Cisco 78-11741-02 User Manual

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Cisco IOS IP Configuration Guide

Release 12.2

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Cisco IOS IP Configuration Guide

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About Cisco IOS Software Documentation xxix

Documentation Objectives xxix

Audience xxix

Documentation Organization xxix

Documentation Modules xxix Master Indexes xxxii Supporting Documents and Resources xxxii

New and Changed Information xxxiii

Document Conventions xxxiii

Obtaining Documentation xxxv

World Wide Web xxxv Documentation CD-ROM xxxv Ordering Documentation xxxv

Documentation Feedback xxxv

Obtaining Technical Assistance xxxvi

Cisco.com xxxvi Technical Assistance Center xxxvi

Contacting TAC by Using the Cisco TAC Website xxxvi Contacting TAC by Telephone xxxvii

CONTENTS

Using Cisco IOS Software xxxix

Understanding Command Modes xxxix

Getting Help xl

Example: How to Find Command Options xli

Using the no and default Forms of Commands xliii

Saving Configuration Changes xliv

Filtering Output from the show and more Commands xliv

Identifying Supported Platforms xlv

Using Feature Navigator xlv Using Software Release Notes xlv

IP Overview IPC-1

IP Addressing and Services IPC-1

IP Routing Protocols IPC-2

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Contents

Determining a Routing Process IPC-2 Interior and Exterior Gateway Protocols IPC-2

Interior Gateway Protocols IPC-3 Exterior Gateway Protocols IPC-3

Multiple Routing Protocols IPC-3

IP Multicast IPC-4

IP ADDRESSING AND SERVICES

Configuring IP Addressing

IPC-7

IP Addressing Task List IPC-7

Assigning IP Addresses to Network Interfaces IPC-7

Assigning Multiple IP Addresses to Network Interfaces IPC-9 Enabling Use of Subnet Zero IPC-9 Disabling Classless Routing Behavior IPC-10 Enabling IP Processing on a Serial Interface IPC-11

Configuring Address Resolution Methods IPC-12

Establishing Address Resolution IPC-12

Defining a Static ARP Cache IPC-13 Setting ARP Encapsulations IPC-14 Enabling Proxy ARP IPC-14 Configuring Local-Area Mobility IPC-15

Mapping Host Names to IP Addresses IPC-15

Assigning Host Names to IP Addresses IPC-16 Specifying the Domain Name IPC-16 Specifying a Name Server IPC-17 Enabling the DNS IPC-17

Using the DNS to Discover ISO CLNS Addresses IPC-17 Configuring HP Probe Proxy Name Requests IPC-18 Configuring the Next Hop Resolution Protocol IPC-18

The Cisco Implementation of NHRP IPC-18

Protocol Operation IPC-20

NHRP Configuration Task List IPC-20

Enabling NHRP on an Interface IPC-21

Configuring a Static IP-to-NBMA Address Mapping for a Station IPC-21

Statically Configuring a Next Hop Server IPC-21

Configuring NHRP Authentication IPC-22

Controlling the Triggering of NHRP IPC-22

Triggering NHRP Based on Traffic Thresholds IPC-23

Controlling the NHRP Packet Rate IPC-25

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Suppressing Forward and Reverse Record Options IPC-26 Specifying the NHRP Responder Address IPC-26 Changing the Time Period NBMA Addresses Are Advertised as Valid IPC-26 Configuring a GRE Tunnel for Multipoint Operation IPC-27 Configuring NHRP Server-Only Mode IPC-27

Enabling IP Routing IPC-27

Routing Assistance When IP Routing Is Disabled IPC-28

Proxy ARP IPC-28 Default Gateway IPC-28 ICMP Router Discovery Protocol IPC-29

Enabling IP Bridging IPC-30

Enabling Integrated Routing and Bridging IPC-30

Configuring a Routing Process IPC-30

Configuring Broadcast Packet Handling IPC-31

Enabling Directed Broadcast-to-Physical Broadcast Translation IPC-31 Forwarding UDP Broadcast Packets and Protocols IPC-32 Establishing an IP Broadcast Address IPC-33 Flooding IP Broadcasts IPC-33

Speeding Up Flooding of UDP Datagrams IPC-34

Contents

Configuring Network Address Translation IPC-35

NAT Applications IPC-35 Benefits IPC-35 NAT Terminology IPC-36 NAT Configuration Task List IPC-36 Translating Inside Source Addresses IPC-37

Configuring Static Translation IPC-38 Configuring Dynamic Translation with an Access List IPC-38

Configuring Dynamic Translation with a Route Map IPC-39 Overloading an Inside Global Address IPC-39 Translating Overlapping Addresses IPC-41

Configuring Static Translation IPC-43

Configuring Dynamic Translation IPC-43 Providing TCP Load Distribution IPC-43 Changing Translation Timeouts IPC-45 Monitoring and Maintaining NAT IPC-46 Deploying NAT Between an IP Phone and Cisco CallManager IPC-46

Monitoring and Maintaining IP Addressing IPC-47

Clearing Caches, Tables, and Databases IPC-47 Specifying the Format of Network Masks IPC-47

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Contents

Displaying System and Network Statistics IPC-48 Monitoring and Maintaining NHRP IPC-49

IP Addressing Examples IPC-49

Creating a Network from Separated Subnets Example IPC-50 Serial Interfaces Configuration Example IPC-50 IP Domains Example IPC-51 Dynamic Lookup Example IPC-51 HP Hosts on a Network Segment Example IPC-51 Logical NBMA Example IPC-51 NHRP over ATM Example IPC-53 Changing the Rate for Triggering SVCs Example IPC-55 Applying NHRP Rates to Specific Destinations Example IPC-57 NHRP on a Multipoint Tunnel Example IPC-58 Broadcasting Examples IPC-59

Flooded Broadcast Example IPC-59

Flooding of IP Broadcasts Example IPC-60 Helper Addresses Example IPC-60 NAT Configuration Examples IPC-61

Dynamic Inside Source Translation Example IPC-61

Overloading Inside Global Addresses Example IPC-62

Translating Overlapping Address Example IPC-62

TCP Load Distribution Example IPC-63 ping Command Example IPC-63

Configuring DHCP IPC-65

DHCP Server Overview IPC-65

DHCP Client Overview IPC-67

DHCP Relay Agent Overview IPC-67

DHCP Configuration Task List IPC-68

Enabling the Cisco IOS DHCP Server and Relay Agent Features IPC-68 Configuring a DHCP Database Agent or Disabling DHCP Conflict Logging IPC-69 Excluding IP Addresses IPC-69 Configuring a DHCP Address Pool IPC-69

Configuring the DHCP Address Pool Name and Entering DHCP Pool Configuration Mode IPC-69

Configuring the DHCP Address Pool Subnet and Mask IPC-70

Configuring the Domain Name for the Client IPC-70

Configuring the IP Domain Name System Servers for the Client IPC-70

Configuring the NetBIOS Windows Internet Naming Service Servers for the Client IPC-70

Configuring the NetBIOS Node Type for the Client IPC-71

Configuring the Default Router for the Client IPC-71

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Configuring the Address Lease Time IPC-71 Configuring Manual Bindings IPC-71 Configuring a DHCP Server Boot File IPC-73 Configuring the Number of Ping Packets IPC-73 Configuring the Timeout Value for Ping Packets IPC-73 Enabling the Cisco IOS DHCP Client on Ethernet Interfaces IPC-73 Configuring DHCP Server Options Import and Autoconfiguration IPC-74 Configuring the Relay Agent Information Option in BOOTREPLY Messages IPC-75 Configuring a Relay Agent Information Reforwarding Policy IPC-75 Enabling the DHCP Smart-Relay Feature IPC-75

Monitoring and Maintaining the DHCP Server IPC-75

Configuration Examples IPC-76

DHCP Database Agent Configuration Example IPC-77 DHCP Address Pool Configuration Example IPC-77 Manual Bindings Configuration Example IPC-78 Cisco IOS DHCP Client Example IPC-78 DHCP Server Options Import and Autoconfiguration Example IPC-79

Contents

Configuring IP Services IPC-81

IP Services Task List IPC-81

Managing IP Connections IPC-81

Enabling ICMP Protocol Unreachable Messages IPC-82 Enabling ICMP Redirect Messages IPC-82 Enabling ICMP Mask Reply Messages IPC-83 Understanding Path MTU Discovery IPC-83 Setting the MTU Packet Size IPC-84 Enabling IP Source Routing IPC-84 Configuring Simplex Ethernet Interfaces IPC-85 Configuring a DRP Server Agent IPC-85

Enabling the DRP Server Agent IPC-86

Limiting the Source of DRP Queries IPC-86

Configuring Authentication of DRP Queries and Responses IPC-86

Filtering IP Packets Using Access Lists IPC-87

Creating Standard and Extended Access Lists Using Numbers IPC-88 Creating Standard and Extended Access Lists Using Names IPC-91 Specifying IP Extended Access Lists with Fragment Control IPC-93

Benefits of Fragment Control in an IP Extended Access List IPC-95 Enabling Turbo Access Control Lists IPC-96

Configuring Turbo ACLs IPC-96

Verifying Turbo ACLs IPC-97

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Applying Time Ranges to Access Lists IPC-97 Including Comments About Entries in Access Lists IPC-98 Applying Access Lists IPC-98

Controlling Access to a Line or Interface IPC-99 Controlling Policy Routing and the Filtering of Routing Information IPC-99 Controlling Dialer Functions IPC-99

Configuring the Hot Standby Router Protocol IPC-100

Enabling HSRP IPC-101 Configuring HSRP Group Attributes IPC-102 Changing the HSRP MAC Refresh Interval IPC-102 Enabling HSRP MIB Traps IPC-103 Enabling HSRP Support for MPLS VPNs IPC-103

Defining VPNs IPC-104 Enabling HSRP IPC-104 Verifying HSRP Support for MPLS VPNs IPC-105

Enabling HSRP Support for ICMP Redirect Messages IPC-105

Redirects to Active HSRP Routers IPC-105 Redirects to Passive HSRP Routers IPC-107 Redirects to Non-HSRP Routers IPC-107 Passive HSRP Router Advertisements IPC-107 Redirects Not Sent IPC-107 Configuring HSRP Support for ICMP Redirect Messages IPC-108

Configuring IP Accounting IPC-108

Configuring IP MAC Accounting IPC-109 Configuring IP Precedence Accounting IPC-110

Configuring TCP Performance Parameters IPC-110

Compressing TCP Packet Headers IPC-111

Expressing TCP Header Compression IPC-111

Changing the Number of TCP Header Compression Connections IPC-112 Setting the TCP Connection Attempt Time IPC-112 Enabling TCP Path MTU Discovery IPC-112 Enabling TCP Selective Acknowledgment IPC-113 Enabling TCP Time Stamp IPC-114 Setting the TCP Maximum Read Size IPC-114 Setting the TCP Window Size IPC-114 Setting the TCP Outgoing Queue Size IPC-115

Configuring IP over WANs IPC-115

Configuring the MultiNode Load Balancing Forwarding Agent IPC-115

MNLB Forwarding Agent Configuration Task List IPC-116

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Enabling CEF IPC-116 Enabling NetFlow Switching IPC-117 Enabling IP Multicast Routing IPC-117 Configuring the Router as a Forwarding Agent IPC-118

Monitoring and Maintaining the IP Network IPC-118

Clearing Caches, Tables, and Databases IPC-118 Monitoring and Maintaining the DRP Server Agent IPC-119 Clearing the Access List Counters IPC-119 Displaying System and Network Statistics IPC-119 Monitoring the MNLB Forwarding Agent IPC-120 Monitoring and Maintaining HSRP Support for ICMP Redirect Messages IPC-120

IP Services Configuration Examples IPC-120

ICMP Services Example IPC-121 Simplex Ethernet Interfaces Example IPC-121 DRP Server Agent Example IPC-122 Numbered Access List Examples IPC-122

Turbo Access Control List Example IPC-123 Implicit Masks in Access Lists Examples IPC-123

Extended Access List Examples IPC-124 Named Access List Example IPC-124 IP Extended Access List with Fragment Control Example IPC-125 Time Range Applied to an IP Access List Example IPC-125 Commented IP Access List Entry Examples IPC-125 IP Accounting Example IPC-126 HSRP Load Sharing Example IPC-126 HSRP MAC Refresh Interval Examples IPC-127

No Switch or Learning Bridge Present Example IPC-127

Switch or Learning Bridge Present Example IPC-127 HSRP MIB Trap Example IPC-128 HSRP Support for MPLS VPNs Example IPC-128 HSRP Support for ICMP Redirect Messages Example IPC-129 MNLB Forwarding Agent Examples IPC-130

Forwarding Agent Configuration for FA2 Example IPC-130

Services Manager Configuration for SM Example IPC-131

Contents

Configuring Server Load Balancing IPC-133

IOS SLB Functions and Capabilities IPC-134

Algorithms for Server Load Balancing IPC-135

Weighted Round Robin IPC-135

Weighted Least Connections IPC-135

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Contents

Port-Bound Servers IPC-136 Client-Assigned Load Balancing IPC-136 Content Flow Monitor Support IPC-136 Sticky Connections IPC-136 Maximum Connections IPC-136 Delayed Removal of TCP Connection Context IPC-137 TCP Session Reassignment IPC-137 Automatic Server Failure Detection IPC-137 Automatic Unfail IPC-137 Slow Start IPC-137 SynGuard IPC-137 Dynamic Feedback Protocol for IOS SLB IPC-138 Alternate IP Addresses IPC-138 Transparent Web Cache Balancing IPC-138 NAT IPC-138 Redundancy Enhancement—Stateless Backup IPC-139

Restrictions IPC-139

IOS SLB Configuration Task List IPC-140

Specifying a Server Farm IPC-141 Specifying a Load-Balancing Algorithm IPC-141 Specifying a Bind ID IPC-142 Specifying a Real Server IPC-142 Configuring Real Server Attributes IPC-142 Enabling the Real Server for Service IPC-143 Specifying a Virtual Server IPC-143 Associating a Virtual Server with a Server Farm IPC-143 Configuring Virtual Server Attributes IPC-143 Adjusting Virtual Server Values IPC-144 Preventing Advertisement of Virtual Server Address IPC-144 Enabling the Virtual Server for Service IPC-144 Configuring IOS SLB Dynamic Feedback Protocol IPC-145 Configuring NAT IPC-145 Implementing IOS SLB Stateless Backup IPC-145

How IOS SLB Stateless Backup Works IPC-145 Configuring IOS SLB Stateless Backup IPC-146 Enabling HSRP IPC-147 Customizing Group Attributes IPC-147 Verifying the IOS SLB Stateless Backup Configuration IPC-147

Verifying IOS SLB IPC-148

Verifying IOS SLB Installation IPC-148

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Verifying Server Failure Detection IPC-149 Troubleshooting IOS SLB IPC-150

Monitoring and Maintaining IOS SLB IPC-151

Configuration Examples IPC-151

IOS SLB Network Configuration Example IPC-152 NAT Configuration Example IPC-153 HSRP Configuration Example IPC-155 IOS SLB Stateless Backup Configuration Example IPC-157

Configuring Mobile IP IPC-159

Mobile IP Overview IPC-159

Why is Mobile IP Needed? IPC-159 Mobile IP Components IPC-160

How Mobile IP Works IPC-161

Agent Discovery IPC-161 Registration IPC-162 Routing IPC-162 Mobile IP Security IPC-163

MN-HA IPC-163

MN-FA IPC-164

FA-HA IPC-164

HA-HA IPC-164 Storing Security Associations IPC-164

Storing SAs on AAA IPC-165 Home Agent Redundancy IPC-165

HSRP Groups IPC-165

How HA Redundancy Works IPC-165

Contents

Prerequisites IPC-166

Mobile IP Configuration Task List IPC-167

Enabling Home Agent Services IPC-167 Enabling Foreign Agent Services IPC-168 Configuring AAA in the Mobile IP Environment IPC-168 Configuring RADIUS in the Mobile IP Environment IPC-169 Configuring TACACS+ in the Mobile IP Environment IPC-169 Verifying Setup IPC-169 Monitoring and Maintaining Mobile IP IPC-170 Shutting Down Mobile IP IPC-170

Mobile IP HA Redundancy Configuration Task List IPC-170

Enabling Mobile IP IPC-171

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Enabling HSRP IPC-171 Configuring HSRP Group Attributes IPC-171 Enabling HA Redundancy for a Physical Network IPC-172 Enabling HA Redundancy for a Virtual Network Using One Physical Network IPC-172 Enabling HA Redundancy for a Virtual Network Using Multiple Physical Networks IPC-173 Enabling HA Redundancy for Multiple Virtual Networks Using One Physical Network IPC-174 Enabling HA Redundancy for Multiple Virtual Networks Using Multiple Physical Networks IPC-174 Verifying HA Redundancy IPC-175 Monitoring and Maintaining HA Redundancy IPC-176

Mobile IP Configuration Examples IPC-176

Home Agent Configuration Example IPC-176 Home Agent Using AAA Server Example IPC-177 Foreign Agent Configuration Example IPC-178 Mobile IP HA Redundancy Configuration Examples IPC-178

HA Redundancy for Physical Networks Example IPC-180 HA Redundancy for a Virtual Network Using One Physical Network Example IPC-182 HA Redundancy for a Virtual Network Using Multiple Physical Networks Example IPC-183 HA Redundancy for Multiple Virtual Networks Using One Physical Network Example IPC-186 HA Redundancy for Multiple Virtual Networks Using Multiple Physical Networks

Example

IPC-189

IP ROUTING PROTOCOLS

Configuring On-Demand Routing

IPC-195

On-Demand Routing Configuration Task List IPC-196 Enabling ODR IPC-196 Filtering ODR Information IPC-197 Redistributing ODR Information into the Dynamic Routing Protocol of the Hub IPC-197 Reconfiguring CDP or ODR Timers IPC-197 Using ODR with Dialer Mappings IPC-198

Configuring Routing Information Protocol IPC-199

RIP Configuration Task List IPC-200

Enabling RIP IPC-200 Allowing Unicast Updates for RIP IPC-201 Applying Offsets to Routing Metrics IPC-201 Adjusting Timers IPC-201 Specifying a RIP Version IPC-202 Enabling RIP Authentication IPC-203 RIP Route Summarization IPC-203

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Restrictions to RIP Route Summarization IPC-205

Configuring Route Summarization on an Interface IPC-205

Verifying IP Route Summarization IPC-205

Disabling Automatic Route Summarization IPC-206 Running IGRP and RIP Concurrently IPC-206 Disabling the Validation of Source IP Addresses IPC-207 Enabling or Disabling Split Horizon IPC-207 Configuring Interpacket Delay IPC-208 Connecting RIP to a WAN IPC-208 RIP Configuration Examples IPC-209 Route Summarization Examples IPC-209

Example 1: Correct Configuration IPC-209

Example 2: Incorrect Configuration IPC-210 Split Horizon Examples IPC-210

Example 1 IPC-210

Example 2 IPC-210 Address Family Timers Example IPC-212

Contents

Configuring IGRP IPC-213

The Cisco IGRP Implementation IPC-213

IGRP Updates IPC-214

IGRP Configuration Task List IPC-214

Creating the IGRP Routing Process IPC-215 Applying Offsets to Routing Metrics IPC-215 Allowing Unicast Updates for IGRP IPC-215 Defining Unequal-Cost Load Balancing IPC-216 Controlling Traffic Distribution IPC-216 Adjusting the IGRP Metric Weights IPC-217 Adjusting Timers IPC-217 Disabling Holddown IPC-218 Enforcing a Maximum Network Diameter IPC-218 Validating Source IP Addresses IPC-218 Enabling or Disabling Split Horizon IPC-219

IGRP Configuration Examples IPC-219

IGRP Feasible Successor Relationship Example IPC-220 Split Horizon Examples IPC-220

Configuring OSPF IPC-223

The Cisco OSPF Implementation IPC-223

OSPF Configuration Task List IPC-224

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Contents

Enabling OSPF IPC-225

Configuring OSPF Interface Parameters IPC-225

Configuring OSPF over Different Physical Networks IPC-226

Configuring Your OSPF Network Type IPC-226 Configuring Point-to-Multipoint, Broadcast Networks IPC-227 Configuring OSPF for Nonbroadcast Networks IPC-227

Configuring OSPF Area Parameters IPC-228

Configuring OSPF NSSA IPC-229

Implementation Considerations IPC-230

Configuring Route Summarization Between OSPF Areas IPC-230

Configuring Route Summarization When Redistributing Routes into OSPF IPC-230

Creating Virtual Links IPC-231

Generating a Default Route IPC-231

Configuring Lookup of DNS Names IPC-232

Forcing the Router ID Choice with a Loopback Interface IPC-232

Controlling Default Metrics IPC-232

Changing the OSPF Administrative Distances IPC-233

Configuring OSPF on Simplex Ethernet Interfaces IPC-233

Configuring Route Calculation Timers IPC-233

Configuring OSPF over On-Demand Circuits IPC-234

Implementation Considerations IPC-235

Logging Neighbors Going Up or Down IPC-235

Changing the LSA Group Pacing IPC-235

Original LSA Behavior IPC-236 LSA Group Pacing With Multiple Timers IPC-236

Blocking OSPF LSA Flooding IPC-237

Reducing LSA Flooding IPC-238

Ignoring MOSPF LSA Packets IPC-238

Displaying OSPF Update Packet Pacing IPC-239

Monitoring and Maintaining OSPF IPC-240

OSPF Configuration Examples IPC-241

OSPF Point-to-Multipoint Example IPC-241 OSPF Point-to-Multipoint, Broadcast Example IPC-243 OSPF Point-to-Multipoint, Nonbroadcast Example IPC-244 Variable-Length Subnet Masks Example IPC-244 OSPF Routing and Route Redistribution Examples IPC-245

Basic OSPF Configuration Examples IPC-245

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Basic OSPF Configuration Example for Internal Router, ABR, and ASBRs IPC-246

Complex Internal Router, ABR, and ASBRs Example IPC-246

Complex OSPF Configuration for ABR Examples IPC-249 Route Map Examples IPC-250 Changing OSPF Administrative Distance Example IPC-252 OSPF over On-Demand Routing Example IPC-253 LSA Group Pacing Example IPC-255 Block LSA Flooding Example IPC-255 Ignore MOSPF LSA Packets Example IPC-255

Configuring EIGRP IPC-257

The Cisco EIGRP Implementation IPC-257

EIGRP Configuration Task List IPC-259

Enabling EIGRP IPC-259 Making the Transition from IGRP to EIGRP IPC-260 Logging EIGRP Neighbor Adjacency Changes IPC-260 Configuring the Percentage of Link Bandwidth Used IPC-260 Adjusting the EIGRP Metric Weights IPC-260

Mismatched K Values IPC-261

The Goodbye Message IPC-262 Applying Offsets to Routing Metrics IPC-262 Disabling Route Summarization IPC-262 Configuring Summary Aggregate Addresses IPC-263 Configuring Floating Summary Routes IPC-263 Configuring EIGRP Route Authentication IPC-265 Configuring EIGRP Protocol-Independent Parameters IPC-266

Adjusting the Interval Between Hello Packets and the Hold Time IPC-266

Disabling Split Horizon IPC-267 Configuring EIGRP Stub Routing IPC-268

Dual-Homed Remote Topology IPC-269 EIGRP Stub Routing Configuration Task List IPC-272

Configuring EIGRP Stub Routing IPC-272

Verifying EIGRP Stub Routing IPC-272

Contents

Monitoring and Maintaining EIGRP IPC-272

EIGRP Configuration Examples IPC-273

Route Summarization Example IPC-273 Route Authentication Example IPC-275 Stub Routing Example IPC-276

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Contents

Configuring Integrated IS-IS IPC-277

IS-IS Configuration Task List IPC-277

Enabling IS-IS and Assigning Areas IPC-277 Enabling IP Routing for an Area on an Interface IPC-279

IS-IS Interface Parameters Configuration Task List IPC-279

Configuring IS-IS Link-State Metrics IPC-280 Setting the Advertised Hello Interval IPC-280 Setting the Advertised CSNP Interval IPC-280 Setting the Retransmission Interval IPC-281 Setting the LSP Transmissions Interval IPC-281 Setting the Retransmission Throttle Interval IPC-281 Setting the Hello Multiplier IPC-282 Specifying Designated Router Election IPC-282 Specifying the Interface Circuit Type IPC-282 Assigning a Password for an Interface IPC-282 Limiting LSP Flooding IPC-283

Blocking Flooding on Specific Interfaces IPC-283 Configuring Mesh Groups IPC-283

Miscellaneous IS-IS Parameters Configuration Task List IPC-284

Generating a Default Route IPC-284 Specifying the System Type IPC-284 Configuring IS-IS Authentication Passwords IPC-285 Summarizing Address Ranges IPC-285 Setting the Overload Bit IPC-285 Changing the Routing Level for an Area IPC-286 Tuning LSP Interval and Lifetime IPC-286 Customizing IS-IS Throttling of LSP Generation, SPF Calculation, and PRC IPC-287

Partial Route Computation (PRC) IPC-287 Benefits of Throttling IS-IS LSP Generation, SPF Calculation, and PRC IPC-287 How Throttling of IS-IS LSP Generation, SPF Calculation, and PRC Works IPC-287

Modifying the Output of show Commands IPC-288

Monitoring IS-IS IPC-289

IS-IS Configuration Examples IPC-289

Enabling IS-IS Configuration Example IPC-289 Multiarea IS-IS Configuration for CLNS Network Example IPC-290 IS-IS Throttle Timers Example IPC-291

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Configuring BGP IPC-293

The Cisco BGP Implementation IPC-293

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How BGP Selects Paths IPC-294

BGP Multipath Support IPC-295

Basic BGP Configuration Task List IPC-295

Advanced BGP Configuration Task List IPC-296

Configuring Basic BGP Features IPC-297

Enabling BGP Routing IPC-297 Configuring BGP Neighbors IPC-297 Managing Routing Policy Changes IPC-298

Resetting a Router Using BGP Dynamic Inbound Soft Reset IPC-299

Resetting a Router Using BGP Outbound Soft Reset IPC-300

Configuring BGP Soft Reset Using Stored Routing Policy Information IPC-300 Verifying BGP Soft Reset IPC-301 Configuring BGP Interactions with IGPs IPC-302 Configuring BGP Weights IPC-303 Disabling Autonomous System Path Comparison IPC-303 Configuring BGP Route Filtering by Neighbor IPC-304 Configuring BGP Filtering Using Prefix Lists IPC-304

How the System Filters Traffic by Prefix List IPC-305

Creating a Prefix List IPC-305

Configuring a Prefix List Entry IPC-306

Configuring How Sequence Numbers of Prefix List Entries Are Specified IPC-306

Deleting a Prefix List or Prefix List Entries IPC-307

Displaying Prefix Entries IPC-307

Clearing the Hit Count Table of Prefix List Entries IPC-308 Configuring BGP Path Filtering by Neighbor IPC-308 Disabling Next Hop Processing on BGP Updates IPC-308

Disabling Next Hop Processing Using a Specific Address IPC-309

Disabling Next Hop Processing Using a Route Map IPC-309

Configuring BGP Next Hop Propagation IPC-309 Configuring the BGP Version IPC-310 Configuring the MED Metric IPC-310

Contents

Configuring Advanced BGP Features IPC-311

Using Route Maps to Modify Updates IPC-311 Resetting eBGP Connections Immediately upon Link Failure IPC-311 Configuring Aggregate Addresses IPC-311 Disabling Automatic Summarization of Network Numbers IPC-312 Configuring BGP Community Filtering IPC-312

Specifying the Format for the Community IPC-314 Configuring BGP Conditional Advertisement IPC-314

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Contents

BGP Conditional Advertisement Configuration Task List IPC-315 Conditional Advertisement of a Set of Routes IPC-315 Verifying BGP Conditional Advertisement IPC-315

BGP Conditional Advertisement Troubleshooting Tips IPC-316 Configuring a Routing Domain Confederation IPC-316 Configuring a Route Reflector IPC-317 Configuring BGP Peer Groups IPC-320

Creating the Peer Group IPC-320

Assigning Options to the Peer Group IPC-321

Making Neighbors Members of the Peer Group IPC-324 Disabling a Peer or Peer Group IPC-324 Indicating Backdoor Routes IPC-325 Modifying Parameters While Updating the IP Routing Table IPC-325 Setting Administrative Distance IPC-325 Adjusting BGP Timers IPC-325 Changing the Default Local Preference Value IPC-326 Redistributing Network 0.0.0.0 IPC-326 Configuring the Router to Consider a Missing MED as Worst Path IPC-327 Selecting Path Based on MEDs from Other Autonomous Systems IPC-327 Configuring the Router to Use the MED to Choose a Path from Subautonomous System

Paths

IPC-327

Configuring the Router to Use the MED to Choose a Path in a Confederation IPC-328 Configuring Route Dampening IPC-328

Minimizing Flapping IPC-328

Understanding Route Dampening Terms IPC-329

Enabling Route Dampening IPC-329

Monitoring and Maintaining BGP Route Dampening IPC-330

xviii

Monitoring and Maintaining BGP IPC-331

Clearing Caches, Tables, and Databases IPC-331 Displaying System and Network Statistics IPC-331 Logging Changes in Neighbor Status IPC-332

BGP Configuration Examples IPC-332

BGP Route Map Examples IPC-333 BGP Neighbor Configuration Examples IPC-336 BGP Prefix List Filtering Examples IPC-337

Route Filtering Configuration Example Using a Single Prefix List IPC-337

Route Filtering Configuration Example Specifying a Group of Prefixes IPC-338

Added or Deleted Prefix List Entries Examples IPC-339 BGP Soft Reset Examples IPC-339

Dynamic Inbound Soft Reset Example IPC-339

Cisco IOS IP Configuration Guide

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Inbound Soft Reset Using Stored Information Example IPC-339 BGP Synchronization Examples IPC-340 BGP Path Filtering by Neighbor Examples IPC-340 BGP Aggregate Route Examples IPC-341 BGP Community with Route Maps Examples IPC-341 BGP Conditional Advertisement Configuration Examples IPC-343 BGP Confederation Examples IPC-344 BGP Peer Group Examples IPC-345

iBGP Peer Group Example IPC-345

eBGP Peer Group Example IPC-345 TCP MD5 Authentication for BGP Examples IPC-346

Configuring Multiprotocol BGP Extensions for IP Multicast IPC-347

Multiprotocol BGP Configuration Task List IPC-349

Understanding NLRI Keywords and Address Families IPC-350 Configuring a Multiprotocol BGP Peer IPC-350 Configuring a Multiprotocol BGP Peer Group IPC-351 Advertising Routes into Multiprotocol BGP IPC-352 Configuring Route Maps for Multiprotocol BGP Prefixes IPC-353 Redistributing Prefixes into Multiprotocol BGP IPC-353 Configuring DVMRP Interoperability with Multiprotocol BGP IPC-354

Redistributing Multiprotocol BGP Routes into DVMRP IPC-354

Redistributing DVMRP Routes into Multiprotocol BGP IPC-355 Configuring a Multiprotocol BGP Route Reflector IPC-356 Configuring Aggregate Multiprotocol BGP Addresses IPC-356 Verifying Multiprotocol BGP Configuration and Operation IPC-357 Multiprotocol BGP Configuration Examples IPC-358 Multiprotocol BGP Peer Examples IPC-359 Multiprotocol BGP Peer Group Examples IPC-359 Multiprotocol BGP Network Advertisement Examples IPC-360 Multiprotocol BGP Route Map Examples IPC-360 Multiprotocol BGP Route Redistribute Examples IPC-360 Multiprotocol BGP Route Reflector Examples IPC-361 Aggregate Multiprotocol BGP Address Examples IPC-361

Contents

Configuring IP Routing Protocol-Independent Features IPC-363

Protocol-Independent Feature Task List IPC-363

Using Variable-Length Subnet Masks IPC-364

Configuring Static Routes IPC-364

Specifying Default Routes IPC-365

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Contents

Specifying a Default Network IPC-365 Understanding Gateway of Last Resort IPC-366

Changing the Maximum Number of Paths IPC-366

Configuring Multi-Interface Load Splitting IPC-366

Redistributing Routing Information IPC-367

Understanding Supported Metric Translations IPC-369

Filtering Routing Information IPC-370

Preventing Routing Updates Through an Interface IPC-370

Configuring Default Passive Interfaces IPC-371 Controlling the Advertising of Routes in Routing Updates IPC-372 Controlling the Processing of Routing Updates IPC-372 Filtering Sources of Routing Information IPC-372

Enabling Policy Routing (PBR) IPC-373

Preverifying Next-Hop Availability IPC-375 Displaying Route-Map Policy Information IPC-376 Enabling Fast-Switched Policy Routing IPC-376 Enabling Local Policy Routing IPC-377

Managing Authentication Keys IPC-377

Monitoring and Maintaining the IP Network IPC-378

Clearing Routes from the IP Routing Table IPC-378 Displaying System and Network Statistics IPC-378

IP Routing Protocol-Independent Configuration Examples IPC-379

Variable-Length Subnet Mask Example IPC-379 Overriding Static Routes with Dynamic Protocols Example IPC-380 Administrative Distance Examples IPC-380 Static Routing Redistribution Example IPC-381 IGRP Redistribution Example IPC-381 RIP and IGRP Redistribution Example IPC-382 EIGRP Redistribution Examples IPC-382 RIP and EIGRP Redistribution Examples IPC-383

Simple Redistribution Example IPC-383

Complex Redistribution Example IPC-383 OSPF Routing and Route Redistribution Examples IPC-384

Basic OSPF Configuration Examples IPC-384

Internal Router, ABR, and ASBRs Configuration Example IPC-385

Complex OSPF Configuration Example IPC-388 Default Metric Values Redistribution Example IPC-390 Policy Routing (Route Map) Examples IPC-390 Passive Interface Examples IPC-392

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Default Passive Interface Example IPC-393 Policy Routing Example IPC-393 Key Management Examples IPC-394

IP MULTICAST

Contents

Configuring IP Multicast Routing

IPC-399

The Cisco IP Multicast Routing Implementation IPC-400

IGMP IPC-400

IGMP Versions IPC-401 PIM IPC-401 CGMP IPC-402

Basic IP Multicast Routing Configuration Task List IPC-402

Advanced IP Multicast Routing Configuration Task List IPC-402

Enabling IP Multicast Routing IPC-403

Enabling PIM on an Interface IPC-403

Enabling Dense Mode IPC-403 Enabling Sparse Mode IPC-404 Enabling Sparse-Dense Mode IPC-404 Configuring PIM Dense Mode State Refresh IPC-405 Configuring a Rendezvous Point IPC-406

Configuring Auto-RP IPC-406

Setting Up Auto-RP in a New Internetwork IPC-407 Adding Auto-RP to an Existing Sparse Mode Cloud IPC-407

Choosing a Default RP IPC-407

Announcing the RP and the Group Range It Serves IPC-407

Assigning the RP Mapping Agent IPC-407

Verifying the Group-to-RP Mapping IPC-408

Starting to Use IP Multicast IPC-408

Preventing Join Messages to False RPs IPC-408 Filtering Incoming RP Announcement Messages IPC-408

IGMP Features Configuration Task List IPC-409

Configuring a Router to Be a Member of a Group IPC-409 Controlling Access to IP Multicast Groups IPC-409 Changing the IGMP Version IPC-410 Modifying the IGMP Host-Query Message and Query Timeout Intervals IPC-410

Routers That Run IGMP Version 1 IPC-410

Routers That Run IGMP Version 2 IPC-411 Configuring IGMP Version 3 IPC-411

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Restrictions IPC-412 Changing the IGMP Query Timeout IPC-413 Changing the Maximum Query Response Time IPC-413 Configuring the Router as a Statically Connected Member IPC-413 Configuring IGMP Leave Latency IPC-414

Configuring the TTL Threshold IPC-415

Disabling Fast Switching of IP Multicast IPC-415

SAP Listener Support Configuration Task List IPC-415

Enabling SAP Listener Support IPC-415 Limiting How Long a SAP Cache Entry Exists IPC-416

Enabling the Functional Address for IP Multicast over Token Ring LANs IPC-416

Configuring PIM Version 2 IPC-417

Prerequisites IPC-418 PIM Version 2 Configuration Task List IPC-418 Specifying the PIM Version IPC-419 Configuring PIM Version 2 Only IPC-419

Configuring PIM Sparse-Dense Mode IPC-419

Defining a PIM Sparse Mode Domain Border Interface IPC-419

Configuring Candidate BSRs IPC-420

Configuring Candidate RPs IPC-420 Making the Transition to PIM Version 2 IPC-421 Deciding When to Configure a BSR IPC-421 Dense Mode IPC-422 Sparse Mode IPC-422 Monitoring the RP Mapping Information IPC-422

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Advanced PIM Features Configuration Task List IPC-422

Understanding PIM Shared Tree and Source Tree (Shortest-Path Tree) IPC-423 Understanding Reverse Path Forwarding IPC-424 Delaying the Use of PIM Shortest-Path Tree IPC-424 Assigning an RP to Multicast Groups IPC-425 Increasing Control over RPs IPC-425 Modifying the PIM Router Query Message Interval IPC-425 Understanding the PIM Registering Process IPC-426

PIM Version 1 Compatibility IPC-426 Limiting the Rate of PIM Register Messages IPC-427 Configuring the IP Source Address of Register Messages IPC-427 Enabling Proxy Registering IPC-427 Enabling PIM Nonbroadcast Multiaccess Mode IPC-428

Configuring an IP Multicast Static Route IPC-429

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Controlling the Transmission Rate to a Multicast Group IPC-430

Configuring RTP Header Compression IPC-430

Enabling RTP Header Compression on a Serial Interface IPC-432 Enabling RTP Header Compression with Frame Relay Encapsulation IPC-432 Changing the Number of Header Compression Connections IPC-432 Enabling Express RTP Header Compression IPC-433

Configuring IP Multicast over ATM Point-to-Multipoint Virtual Circuits IPC-434

Enabling IP Multicast over ATM Point-to-Multipoint VCs IPC-436 Limiting the Number of VCs IPC-436

Idling Policy IPC-437 How the Idling Policy Works IPC-437 Keeping VCs from Idling IPC-437

Configuring an IP Multicast Boundary IPC-438

Configuring an Intermediate IP Multicast Helper IPC-438

Contents

Storing IP Multicast Headers IPC-439

Enabling CGMP IPC-440

Configuring Stub IP Multicast Routing IPC-440

Load Splitting IP Multicast Traffic Across Equal-Cost Paths Configuration Task List IPC-441

Enabling Native Load Splitting IPC-442 Enabling Load Splitting Across Tunnels IPC-442

Configuring the Access Router IPC-443 Configuring the Router at the Opposite End of the Tunnel IPC-443 Configuring Both Routers to RPF IPC-444 Verifying the Load Splitting IPC-445

Monitoring and Maintaining IP Multicast Routing Configuration Task List IPC-445

Clearing Caches, Tables, and Databases IPC-446 Displaying System and Network Statistics IPC-446 Using IP Multicast Heartbeat IPC-447

IP Multicast Configuration Examples IPC-448

PIM Dense Mode Example IPC-448 PIM Sparse Mode Example IPC-448 PIM Dense Mode State Refresh Example IPC-449 Functional Address for IP Multicast over Token Ring LAN Example IPC-449 PIM Version 2 Examples IPC-449

BSR Configuration Example IPC-449 Border Router Configuration Example IPC-450 RFC 2362 Interoperable Candidate RP Example IPC-450

RTP Header Compression Examples IPC-451

Express RTP Header Compression with PPP Encapsulation Example IPC-452

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Contents

Express RTP Header Compression with Frame Relay Encapsulation Example IPC-453 IP Multicast over ATM Point-to-Multipoint VC Example IPC-454 Administratively Scoped Boundary Example IPC-455 IP Multicast Helper Example IPC-455 Stub IP Multicast Example IPC-456 Load Splitting IP Multicast Traffic Across Equal-Cost Paths Example IPC-457 IP Multicast Heartbeat Example IPC-458

Configuring Source Specific Multicast IPC-459

SSM Components Overview IPC-459

How SSM Differs from Internet Standard Multicast IPC-460

SSM IP Address Range IPC-460

SSM Operations IPC-460

IGMPv3 Host Signalling IPC-461

IGMP v3lite Host Signalling IPC-461

URD Host Signalling IPC-462

Benefits IPC-464

IP Multicast Address Management Not Required IPC-464 Denial of Service Attacks from Unwanted Sources Inhibited IPC-464 Easy to Install and Manage IPC-464 Ideal for Internet Broadcast Applications IPC-465

Restrictions IPC-465

Legacy Applications Within the SSM Range Restrictions IPC-465 IGMP v3lite and URD Require a Cisco IOS Last Hop Router IPC-465 Address Management Restrictions IPC-465 IGMP Snooping and CGMP Limitations IPC-466 URD Intercept URL Limitations IPC-466 State Maintenance Limitations IPC-466 HSIL Limitations IPC-466

SSM Configuration Task List IPC-467

Configuring SSM IPC-467 Monitoring SSM IPC-467

SSM Configuration Examples IPC-468

SSM with IGMPv3 Example IPC-468 SSM with IGMP v3lite and URD Example IPC-468 SSM Filtering Example IPC-468

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Configuring Bidirectional PIM IPC-471

Bidir-PIM Overview IPC-471

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DF Election IPC-473 Bidirectional Group Tree Building IPC-474 Packet Forwarding IPC-474

Bidir-PIM Configuration Task List IPC-474

Prerequisites IPC-474 Configuring Bidir-PIM IPC-475 Verifying Bidirectional Groups IPC-475 Monitoring and Maintaining Bidir-PIM IPC-476

Bidir-PIM Configuration Example IPC-476

Configuring Multicast Source Discovery Protocol IPC-477

How MSDP Works IPC-477

Benefits IPC-479

Prerequisites IPC-479

MSDP Configuration Task List IPC-479

Configuring an MSDP Peer IPC-480 Caching SA State IPC-480 Requesting Source Information from an MSDP Peer IPC-481 Controlling Source Information That Your Router Originates IPC-481

Redistributing Sources IPC-481 Filtering SA Request Messages IPC-482

Controlling Source Information That Your Router Forwards IPC-482

Using an MSDP Filter IPC-482

Using TTL to Limit the Multicast Data Sent in SA Messages IPC-483 Controlling Source Information That Your Router Receives IPC-483 Configuring a Default MSDP Peer IPC-484 Configuring an MSDP Mesh Group IPC-485 Shutting Down an MSDP Peer IPC-485 Including a Bordering PIM Dense Mode Region in MSDP IPC-486 Configuring an Originating Address Other Than the RP Address IPC-486

Contents

Monitoring and Maintaining MSDP IPC-487

MSDP Configuration Examples IPC-488

Default MSDP Peer IPC-488 Logical RP IPC-488

Configuring PGM Host and Router Assist IPC-493

PGM Overview IPC-493

PGM Host Configuration Task List IPC-495

Prerequisites IPC-495

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Contents

Enabling PGM Host IPC-495

Enabling PGM Host with a Virtual Host Interface IPC-496 Enabling PGM Host with a Physical Interface IPC-496

Verifying PGM Host Configuration IPC-496

PGM Router Assist Configuration Task List IPC-498

Prerequisites IPC-498 Enabling PGM Router Assist IPC-498

Enabling PGM Router Assist with a Virtual Host Interface IPC-499 Enabling PGM Router Assist with a Physical Interface IPC-499

Monitoring and Maintaining PGM Host and Router Assist IPC-499

Monitoring and Maintaining PGM Host IPC-499 Monitoring and Maintaining PGM Router Assist IPC-500

PGM Host and Router Assist Configuration Examples IPC-500

PGM Host with a Virtual Interface Example IPC-501 PGM Host with a Physical Interface Example IPC-501 PGM Router Assist with a Virtual Interface Example IPC-502 PGM Router Assist with a Physical Interface Example IPC-502

Configuring Unidirectional Link Routing IPC-505

UDLR Overview IPC-505

UDLR Tunnel IPC-506 IGMP UDLR IPC-506 IGMP Proxy IPC-507

UDLR Tunnel Configuration Task List IPC-508

Prerequisite IPC-508 Configuring UDLR Tunnel IPC-508

IGMP UDLR Configuration Task List IPC-510

Prerequisites IPC-510 Configuring the IGMP UDL IPC-510 Changing the Distance for the Default RPF Interface IPC-511 Monitoring IGMP UDLR IPC-511

IGMP Proxy Configuration Task List IPC-511

Prerequisites IPC-512 Configuring IGMP Proxy IPC-512 Verifying IGMP Proxy IPC-512

UDLR Configuration Examples IPC-513

UDLR Tunnel Example IPC-513 IGMP UDLR Example IPC-514 IGMP Proxy Example IPC-516

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Integrated UDLR Tunnel, IGMP UDLR, and IGMP Proxy Example IPC-518

Using IP Multicast Tools IPC-521

Multicast Routing Monitor Overview IPC-521

Benefits IPC-521 Restrictions IPC-522

MRM Configuration Task List IPC-522

Configuring a Test Sender and Test Receiver IPC-522

Monitoring Multiple Groups IPC-523 Configuring a Manager IPC-524 Conducting an MRM Test IPC-524

Monitoring IP Multicast Routing IPC-525

Monitoring and Maintaining MRM IPC-525

MRM Configuration Example IPC-526

Contents

Configuring Router-Port Group Management Protocol IPC-527

IP Multicast Routing Overview IPC-527

RGMP Overview IPC-528

RGMP Configuration Task List IPC-531

Prerequisites IPC-531 Enabling RGMP IPC-532 Verifying RGMP Configuration IPC-532

Monitoring and Maintaining RGMP IPC-533

RGMP Configuration Example IPC-534

Configuring DVMRP Interoperability IPC-537

Basic DVMRP Interoperability Configuration Task List IPC-537

Configuring DVMRP Interoperability IPC-538

Responding to mrinfo Requests IPC-538 Configuring a DVMRP Tunnel IPC-539 Advertising Network 0.0.0.0 to DVMRP Neighbors IPC-540

Advanced DVMRP Interoperability Configuration Task List IPC-540

Enabling DVMRP Unicast Routing IPC-540 Limiting the Number of DVMRP Routes Advertised IPC-541 Changing the DVMRP Route Threshold IPC-541 Configuring a DVMRP Summary Address IPC-541 Disabling DVMRP Automatic summarization IPC-542 Adding a Metric Offset to the DVMRP Route IPC-542 Rejecting a DVMRP Nonpruning Neighbor IPC-543 Configuring a Delay Between DVRMP Reports IPC-544

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Contents

Monitoring and Maintaining DVMRP IPC-545

DVMRP Configuration Examples IPC-545

DVMRP Interoperability Example IPC-545 DVMRP Tunnel Example IPC-545

INDEX

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About Cisco IOS Software Documentation

This chapter discusses the objectives, audience, organization, and conventions of Cisco IOS software documentation. It also provides sources for obtaining documentation from Cisco Systems.

Documentation Objectives

Cisco IOS software documentation describes the tasks and commands necessary to configure and maintain Cisco networking devices.

Audience

The Cisco IOS software documentation set is intended primarily for users who configure and maintain Cisco networking devices (such as routers and switches) but who may not be familiar with the tasks, the relationship between tasks, or the Cisco IOS software commands necessary to perform particular tasks. The Cisco IOS software documentation set is also intended for those users experienced with Cisco IOS software who need to know about new features, new configuration options, and new software characteristics in the current Cisco IOS software release.

Documentation Organization

The Cisco IOS software documentation set consists of documentation modules and master indexes. In addition to the main documentation set, there are supporting documents and resources.

Documentation Modules

The Cisco IOS documentation modules consist of configuration guides and corresponding command reference publications. Chapters in a configuration guide describe protocols, configuration tasks, and Cisco IOS software functionality and contain comprehensive configuration examples. Chapters in a command reference publication provide complete Cisco IOS command syntax information. Use each configuration guide in conjunction with its corresponding command reference publication.

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Documentation Organization

Figure 1 shows the Cisco IOS software documentation modules.

Note The abbreviations (for example, FC and FR) next to the book icons are page designators,

which are defined in a key in the index of each document to help you with navigation. The bullets under each module list the major technology areas discussed in the corresponding books.

Figure 1 Cisco IOS Software Documentation Modules

About Cisco IOS Software Documentation

Cisco IOS

Configuration Fundamentals Configuration Guide

Cisco IOS Configuration Fundamentals

Command Reference

Module FC/FR:

• Cisco IOS User Interfaces

• File Management

• System Management

Cisco IOS Wide-Area Networking Configuration Guide

IPC

Cisco IOS IP Configuration Guide

IP2R

Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols

Module IPC/IP1R/IP2R/IP3R:

• IP Addressing and Services

• IP Routing Protocols

• IP Multicast

Cisco IOS Interface Configuration Guide

Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services

Cisco IOS IP Command Reference, Volume 3 of 3: Multicast

IP1R

IP3R

MWC

Cisco IOS

P2C

AppleTalk and Novell IPX Configuration Guide

P2R

Cisco IOS Mobile Wireless Configuration Guide

Cisco IOS AppleTalk and Novell IPX Command Reference

Module P2C/P2R:

• AppleTalk

• Novell IPX

Cisco IOS

P3C

Apollo Domain, Banyan VINES, DECnet, ISO CLNS, and XNS Configuration Guide

P3R

Module P3C/P3R:

• Apollo Domain

• Banyan VINES

• DECnet

• ISO CLNS

• XNS

Cisco IOS Security Configuration Guide

Cisco IOS Apollo Domain, Banyan VINES, DECnet, ISO CLNS, and XNS Command Reference

Cisco IOS Wide-Area Networking

Command Reference

Module WC/WR:

• ATM

• Broadband Access

• Frame Relay

• SMDS

• X.25 and LAPB

xxx

Cisco IOS Interface Command

Reference

Module IC/IR:

• LAN Interfaces

• Serial Interfaces

• Logical Interfaces

Cisco IOS IP Configuration Guide

Cisco IOS Mobile Wireless

MWR

Command Reference

Module MWC/MWR:

• General Packet Radio Service

Cisco IOS Security Command

Reference

Module SC/SR:

• AAA Security Services

• Security Server Protocols

• Traffic Filtering and Firewalls

• IP Security and Encryption

• Passwords and Privileges

• Neighbor Router Authentication

• IP Security Options

• Supported AV Pairs

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About Cisco IOS Software Documentation

Documentation Organization

Cisco IOS

Dial Technologies Configuration Guide

Cisco IOS Dial

Technologies Command Reference

Module DC/DR:

• Preparing for Dial Access

• Modem and Dial Shelf Configuration and Management

• ISDN Configuration

• Signalling Configuration

• Dial-on-Demand Routing Configuration

• Dial-Backup Configuration

• Dial-Related Addressing Services

• Virtual Templates, Profiles, and Networks

• PPP Configuration

• Callback and Bandwidth Allocation Configuration

• Dial Access Specialized Features

• Dial Access Scenarios

Cisco IOS Terminal Services Configuration Guide

Cisco IOS

Terminal Services Command Reference

Module TC/TR:

• ARA

• LAT

• NASI

• Telnet

• TN3270

• XRemote

• X.28 PAD

• Protocol Translation

Cisco IOS Bridging and IBM Networking Configuration Guide

B1R

Cisco IOS Bridging and IBM Networking Command Reference, Volume 1 of 2

Module BC/B1R:

• Transparent Bridging

• SRB

• Token Ring Inter-Switch Link

• Token Ring Route Switch Module

• RSRB

• DLSw+

• Serial Tunnel and Block Serial Tunnel

• LLC2 and SDLC

• IBM Network Media Translation

• SNA Frame Relay Access

• NCIA Client/Server

• Airline Product Set

B2R

Cisco IOS Bridging and IBM Networking Command Reference, Volume 2 of 2

Module BC/B2R:

• DSPU and SNA Service Point

• SNA Switching Services

• Cisco Transaction Connection

• Cisco Mainframe Channel Connection

• CLAW and TCP/IP Offload

• CSNA, CMPC, and CMPC+

• TN3270 Server

Cisco IOS

Voice, Video, and Fax Configuration Guide

Cisco IOS Voice, Video, and Fax Command Reference

Module VC/VR:

• Voice over IP

• Call Control Signalling

• Voice over Frame Relay

• Voice over ATM

• Telephony Applications

• Trunk Management

• Fax, Video, and Modem Support

Cisco IOS Quality of Service Solutions Configuration Guide

Cisco IOS Quality of Service Solutions Command Reference

Module QC/QR:

• Packet Classification

• Congestion Management

• Congestion Avoidance

• Policing and Shaping

• Signalling

• Link Efficiency Mechanisms

Cisco IOS Switching Services Configuration Guide

Cisco IOS Switching Services Command

Reference

Module XC/XR:

• Cisco IOS Switching Paths

• NetFlow Switching

• Multiprotocol Label Switching

• Multilayer Switching

• Multicast Distributed Switching

• Virtual LANs

• LAN Emulation

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Documentation Organization

Master Indexes

Two master indexes provide indexing information for the Cisco IOS software documentation set: an index for the configuration guides and an index for the command references. Individual books also contain a book-specific index.

The master indexes provide a quick way for you to find a command when you know the command name but not which module contains the command. When you use the online master indexes, you can click the page number for an index entry and go to that page in the online document.

Supporting Documents and Resources

The following documents and resources support the Cisco IOS software documentation set:

• Cisco IOS Command Summary (two volumes)—This publication explains the function and syntax

of the Cisco IOS software commands. For more information about defaults and usage guidelines, refer to the Cisco IOS command reference publications.

• Cisco IOS System Error Messages—This publication lists and describes Cisco IOS system error

messages. Not all system error messages indicate problems with your system. Some are purely informational, and others may help diagnose problems with communications lines, internal hardware, or the system software.

About Cisco IOS Software Documentation

• Cisco IOS Debug Command Reference—This publication contains an alphabetical listing of the

debug commands and their descriptions. Documentation for each command includes a brief description of its use, command syntax, usage guidelines, and sample output.

• Dictionary of Internetworking Terms and Acronyms—This Cisco publication compiles and defines

the terms and acronyms used in the internetworking industry.

• New feature documentation—The Cisco IOS software documentation set documents the mainline

release of Cisco IOS software (for example, Cisco IOS Release 12.2). New software features are introduced in early deployment releases (for example, the Cisco IOS “T” release train for 12.2,

12.2(x)T). Documentation for these new features can be found in standalone documents called “feature modules.” Feature module documentation describes new Cisco IOS software and hardware networking functionality and is available on Cisco.com and the Documentation CD-ROM.

• Release notes—This documentation describes system requirements, provides information about

new and changed features, and includes other useful information about specific software releases. See the section “Using Software Release Notes” in the chapter “Using Cisco IOS Software” for more information.

• Caveats documentation—This documentation provides information about Cisco IOS software

defects in specific software releases.

• RFCs—RFCs are standards documents maintained by the Internet Engineering Task Force (IETF).

Cisco IOS software documentation references supported RFCs when applicable. The full text of referenced RFCs may be obtained on the World Wide Web at http://www.rfc-editor.org/.

• MIBs—MIBs are used for network monitoring. For lists of supported MIBs by platform and

release, and to download MIB files, see the Cisco MIB website on Cisco.com at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.

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About Cisco IOS Software Documentation

New and Changed Information

The following is new or changed information since the last release of the Cisco IOS IP and IP routing publications:

• The title of the Cisco IOS IP and IP Routing Configuration Guide has been changed to Cisco IOS

IP Configuration Guide.

• The Cisco IOS IP and IP Routing Command Reference has been divided into three separate

publications with the following titles:

–

Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services

–

Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols

–

Cisco IOS IP Command Reference, Volume 3 of 3: Multicast

• The following new chapters were added to the Cisco IOS IP Configuration Guide:

–

“Configuring Server Load Balancing”

–

“Configuring Source Specific Multicast”

–

“Configuring Bidirectional PIM”

–

“Configuring Router-Port Group Management Protocol”

• The following new chapter was added to the Cisco IOS IP Command Reference, Volume 1 of 3:

Addressing and Services:

New and Changed Information

–

“Server Load Balancing Commands”

Document Conventions

Within Cisco IOS software documentation, the term router is generally used to refer to a variety of Cisco products (for example, routers, access servers, and switches). Routers, access servers, and other networking devices that support Cisco IOS software are shown interchangeably within examples. These products are used only for illustrative purposes; that is, an example that shows one product does not necessarily indicate that other products are not supported.

The Cisco IOS documentation set uses the following conventions:

Convention Description

^ or Ctrl The ^ and Ctrl symbols represent the Control key. For example, the key combination ^D or Ctrl-D

means hold down the Control key while you press the D key. Keys are indicated in capital letters but are not case sensitive.

string A string is a nonquoted set of characters shown in italics. For example, when setting an SNMP

community string to public, do not use quotation marks around the string or the string will include the quotation marks.

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About Cisco IOS Software Documentation

Document Conventions

Command syntax descriptions use the following conventions:

Convention Description

boldface Boldface text indicates commands and keywords that you enter literally as shown.

italics Italic text indicates arguments for which you supply values.

[x] Square brackets enclose an optional element (keyword or argument).

| A vertical line indicates a choice within an optional or required set of keywords or arguments.

[x | y] Square brackets enclosing keywords or arguments separated by a vertical line indicate an optional

choice.

{x | y} Braces enclosing keywords or arguments separated by a vertical line indicate a required choice.

Nested sets of square brackets or braces indicate optional or required choices within optional or required elements. For example:

Convention Description

[x {y | z}] Braces and a vertical line within square brackets indicate a required choice within an optional element.

Examples use the following conventions:

Convention Description

screen

boldface screen

Examples of information displayed on the screen are set in Courier font.

Examples of text that you must enter are set in Courier bold font.

< > Angle brackets enclose text that is not printed to the screen, such as passwords.

! An exclamation point at the beginning of a line indicates a comment line. (Exclamation points are also

displayed by the Cisco IOS software for certain processes.)

[ ] Square brackets enclose default responses to system prompts.

The following conventions are used to attract the attention of the reader:

Caution Means reader be careful. In this situation, you might do something that could result in

equipment damage or loss of data.

Note Means reader take note. Notes contain helpful suggestions or references to materials not

contained in this manual.

Timesaver Means the described action saves time. You can save time by performing the action

described in the paragraph.

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About Cisco IOS Software Documentation

Obtaining Documentation

The following sections provide sources for obtaining documentation from Cisco Systems.

World Wide Web

The most current Cisco documentation is available on the World Wide Web at the following website:

http://www.cisco.com

Translated documentation is available at the following website:

http://www.cisco.com/public/countries_languages.html

Documentation CD-ROM

Cisco documentation and additional literature are available in a CD-ROM package, which ships with your product. The Documentation CD-ROM is updated monthly and may be more current than printed documentation. The CD-ROM package is available as a single unit or through an annual subscription.

Obtaining Documentation

Ordering Documentation

Cisco documentation can be ordered in the following ways:

• Registered Cisco Direct Customers can order Cisco product documentation from the Networking

Products MarketPlace:

http://www.cisco.com/cgi-bin/order/order_root.pl

• Registered Cisco.com users can order the Documentation CD-ROM through the online

Subscription Store:

http://www.cisco.com/go/subscription

• Nonregistered Cisco.com users can order documentation through a local account representative by

calling Cisco corporate headquarters (California, USA) at 408 526-7208 or, in North America, by calling 800 553-NETS(6387).

Documentation Feedback

If you are reading Cisco product documentation on the World Wide Web, you can submit technical comments electronically. Click Feedback in the toolbar and select Documentation. After you complete the form, click Submit to send it to Cisco.

You can e-mail your comments to bug-doc@cisco.com.

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Obtaining Technical Assistance

To submit your comments by mail, use the response card behind the front cover of your document, or write to the following address:

Cisco Systems, Inc. Document Resource Connection 170 West Tasman Drive San Jose, CA 95134-9883

We appreciate your comments.

Obtaining Technical Assistance

Cisco provides Cisco.com as a starting point for all technical assistance. Customers and partners can obtain documentation, troubleshooting tips, and sample configurations from online tools. For Cisco.com registered users, additional troubleshooting tools are available from the TAC website.

Cisco.com

About Cisco IOS Software Documentation

Cisco.com is the foundation of a suite of interactive, networked services that provides immediate, open access to Cisco information and resources at anytime, from anywhere in the world. This highly integrated Internet application is a powerful, easy-to-use tool for doing business with Cisco.

Cisco.com provides a broad range of features and services to help customers and partners streamline business processes and improve productivity. Through Cisco.com, you can find information about Cisco and our networking solutions, services, and programs. In addition, you can resolve technical issues with online technical support, download and test software packages, and order Cisco learning materials and merchandise. Valuable online skill assessment, training, and certification programs are also available.

Customers and partners can self-register on Cisco.com to obtain additional personalized information and services. Registered users can order products, check on the status of an order, access technical support, and view benefits specific to their relationships with Cisco.

To access Cisco.com, go to the following website:

http://www.cisco.com

Technical Assistance Center

The Cisco TAC website is available to all customers who need technical assistance with a Cisco product or technology that is under warranty or covered by a maintenance contract.

Contacting TAC by Using the Cisco TAC Website

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If you have a priority level 3 (P3) or priority level 4 (P4) problem, contact TAC by going to the TAC website:

http://www.cisco.com/tac

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About Cisco IOS Software Documentation

P3 and P4 level problems are defined as follows:

• P3—Your network performance is degraded. Network functionality is noticeably impaired, but

most business operations continue.

• P4—You need information or assistance on Cisco product capabilities, product installation, or basic

product configuration.

In each of the above cases, use the Cisco TAC website to quickly find answers to your questions.

To register for Cisco.com, go to the following website:

http://www.cisco.com/register/

If you cannot resolve your technical issue by using the TAC online resources, Cisco.com registered users can open a case online by using the TAC Case Open tool at the following website:

http://www.cisco.com/tac/caseopen

Contacting TAC by Telephone

If you have a priority level 1 (P1) or priority level 2 (P2) problem, contact TAC by telephone and immediately open a case. To obtain a directory of toll-free numbers for your country, go to the following website:

http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml

P1 and P2 level problems are defined as follows:

Obtaining Technical Assistance

• P1—Your production network is down, causing a critical impact to business operations if service

is not restored quickly. No workaround is available.

• P2—Your production network is severely degraded, affecting significant aspects of your business

operations. No workaround is available.

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Obtaining Technical Assistance

About Cisco IOS Software Documentation

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Using Cisco IOS Software

This chapter provides helpful tips for understanding and configuring Cisco IOS software using the command-line interface (CLI). It contains the following sections:

• Understanding Command Modes

• Getting Help

• Using the no and default Forms of Commands

• Saving Configuration Changes

• Filtering Output from the show and more Commands

• Identifying Supported Platforms

For an overview of Cisco IOS software configuration, refer to the Cisco IOS Configuration Fundamentals Configuration Guide.

For information on the conventions used in the Cisco IOS software documentation set, see the chapter “About Cisco IOS Software Documentation” located at the beginning of this book.

Understanding Command Modes

You use the CLI to access Cisco IOS software. Because the CLI is divided into many different modes, the commands available to you at any given time depend on the mode you are currently in. Entering a question mark (?) at the CLI prompt allows you to obtain a list of commands available for each command mode.

When you log in to the CLI, you are in user EXEC mode. User EXEC mode contains only a limited subset of commands. To have access to all commands, you must enter privileged EXEC mode, normally by using a password. From privileged EXEC mode you can issue any EXEC command—user or privileged mode—or you can enter global configuration mode. Most EXEC commands are one-time commands. For example, show commands show important status information, and clear commands clear counters or interfaces. The EXEC commands are not saved when the software reboots.

Configuration modes allow you to make changes to the running configuration. If you later save the running configuration to the startup configuration, these changed commands are stored when the software is rebooted. To enter specific configuration modes, you must start at global configuration mode. From global configuration mode, you can enter interface configuration mode and a variety of other modes, such as protocol-specific modes.

ROM monitor mode is a separate mode used when the Cisco IOS software cannot load properly. If a valid software image is not found when the software boots or if the configuration file is corrupted at startup, the software might enter ROM monitor mode.

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Getting Help

Table 1 describes how to access and exit various common command modes of the Cisco IOS software. It also shows examples of the prompts displayed for each mode.

Table 1 Accessing and Exiting Command Modes

Command Mode Access Method Prompt Exit Method

User EXEC Log in.

Privileged EXEC

From user EXEC mode, use the enable EXEC

Router>

Router#

Use the logout command.

To return to user EXEC mode, use the disable command.

command.

Global configuration

From privileged EXEC mode, use the configure terminal privileged

Router(config)#

To return to privileged EXEC mode from global configuration mode, use the exit or end command, or press Ctrl-Z.

EXEC command.

Interface configuration

From global configuration mode, specify an interface using an interface command.

ROM monitor From privileged EXEC

mode, use the reload

Router(config-if)#

To return to global configuration mode, use the exit command.

To return to privileged EXEC mode, use the end command, or press Ctrl-Z.

To exit ROM monitor mode, use the continue

command. EXEC command. Press the Break key during the first 60 seconds while the system is booting.

Using Cisco IOS Software

For more information on command modes, refer to the “Using the Command-Line Interface” chapter in the Cisco IOS Configuration Fundamentals Configuration Guide.

Getting Help

Entering a question mark (?) at the CLI prompt displays a list of commands available for each command mode. You can also get a list of keywords and arguments associated with any command by using the context-sensitive help feature.

To get help specific to a command mode, a command, a keyword, or an argument, use one of the following commands:

Command Purpose

help

abbreviated-command-entry?

abbreviated-command-entry<Tab>

command ?

Provides a brief description of the help system in any command mode.

Provides a list of commands that begin with a particular character string. (No space between command and question mark.)

Completes a partial command name.

Lists all commands available for a particular command mode.

Lists the keywords or arguments that you must enter next on the command line. (Space between command and question mark.)

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Using Cisco IOS Software

Example: How to Find Command Options

This section provides an example of how to display syntax for a command. The syntax can consist of optional or required keywords and arguments. To display keywords and arguments for a command, enter a question mark (?) at the configuration prompt or after entering part of a command followed by a space. The Cisco IOS software displays a list and brief description of available keywords and arguments. For example, if you were in global configuration mode and wanted to see all the keywords or arguments for the arap command, you would type arap ?.

The <cr> symbol in command help output stands for “carriage return.” On older keyboards, the carriage return key is the Return key. On most modern keyboards, the carriage return key is the Enter key. The <cr> symbol at the end of command help output indicates that you have the option to press Enter to complete the command and that the arguments and keywords in the list preceding the <cr> symbol are optional. The <cr> symbol by itself indicates that no more arguments or keywords are available and that you must press Enter to complete the command.

Table 2 shows examples of how you can use the question mark (?) to assist you in entering commands. The table steps you through configuring an IP address on a serial interface on a Cisco 7206 router that is running Cisco IOS Release 12.0(3).

Table 2 How to Find Command Options

Getting Help

Command Comment

Router> enable Password: <password> Router#

Enter the enable command and password to access privileged EXEC commands. You are in privileged EXEC mode when the prompt changes to

Router#.

Router# configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router(config)#

Enter the configure terminal privileged EXEC command to enter global configuration mode. You are in global configuration mode when the prompt changes to

Router(config)# interface serial ?

<0-6> Serial interface number

Router(config)# interface serial 4 ?

Router(config)# interface serial 4/ ?

<0-3> Serial interface number Router(config)# interface serial 4/0 Router(config-if)#

Enter interface configuration mode by specifying the serial interface that you want to configure using the interface serial global configuration command.

Enter ? to display what you must enter next on the command line. In this example, you must enter the serial interface slot number and port number, separated by a forward slash.

You are in interface configuration mode when the prompt changes to

Router(config-if)#.

Router(config)#.

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Getting Help

Table 2 How to Find Command Options (continued)

Command Comment

Router(config-if)# ? Interface configuration commands:

. . . ip Interface Internet Protocol config commands keepalive Enable keepalive lan-name LAN Name command llc2 LLC2 Interface Subcommands load-interval Specify interval for load calculation for an

interface locaddr-priority Assign a priority group logging Configure logging for interface loopback Configure internal loopback on an interface mac-address Manually set interface MAC address mls mls router sub/interface commands mpoa MPOA interface configuration commands mtu Set the interface Maximum Transmission Unit (MTU) netbios Use a defined NETBIOS access list or enable

name-caching no Negate a command or set its defaults nrzi-encoding Enable use of NRZI encoding ntp Configure NTP . . .

Router(config-if)#

Router(config-if)# ip ? Interface IP configuration subcommands:

access-group Specify access control for packets accounting Enable IP accounting on this interface address Set the IP address of an interface authentication authentication subcommands bandwidth-percent Set EIGRP bandwidth limit broadcast-address Set the broadcast address of an interface cgmp Enable/disable CGMP directed-broadcast Enable forwarding of directed broadcasts dvmrp DVMRP interface commands hello-interval Configures IP-EIGRP hello interval helper-address Specify a destination address for UDP broadcasts hold-time Configures IP-EIGRP hold time . . .

Router(config-if)# ip

Enter ? to display a list of all the interface configuration commands available for the serial interface. This example shows only some of the available interface configuration commands.

Enter the command that you want to configure for the interface. This example uses the ip command.

Enter ? to display what you must enter next on the command line. This example shows only some of the available interface IP configuration commands.

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Using Cisco IOS Software

Using the no and default Forms of Commands

Table 2 How to Find Command Options (continued)

Command Comment

Router(config-if)# ip address ?

A.B.C.D IP address negotiated IP Address negotiated over PPP

Router(config-if)# ip address

Enter the command that you want to configure for the interface. This example uses the ip address command.

Enter ? to display what you must enter next on the command line. In this example, you must enter an IP address or the negotiated keyword.

A carriage return (<cr>) is not displayed; therefore, you must enter additional keywords or arguments to complete the command.

Router(config-if)# ip address 172.16.0.1 ?

A.B.C.D IP subnet mask

Router(config-if)# ip address 172.16.0.1

Enter the keyword or argument you want to use. This example uses the

172.16.0.1 IP address.

Enter ? to display what you must enter next on the command line. In this example, you must enter an IP subnet mask.

A <cr> is not displayed; therefore, you must enter additional keywords or arguments to complete the command.

Router(config-if)# ip address 172.16.0.1 255.255.255.0 ?

secondary Make this IP address a secondary address <cr>

Router(config-if)# ip address 172.16.0.1 255.255.255.0

Enter the IP subnet mask. This example uses the 255.255.255.0 IP subnet mask.

Enter ? to display what you must enter next on the command line. In this example, you can enter the secondary keyword, or you can press Enter.

A <cr> is displayed; you can press Enter to complete the command, or you can enter another keyword.

Router(config-if)# ip address 172.16.0.1 255.255.255.0 Router(config-if)#

In this example, Enter is pressed to complete the command.

Using the no and default Forms of Commands

Almost every configuration command has a no form. In general, use the no form to disable a function. Use the command without the no keyword to reenable a disabled function or to enable a function that is disabled by default. For example, IP routing is enabled by default. To disable IP routing, use the no ip routing command; to reenable IP routing, use the ip routing command. The Cisco IOS software command reference publications provide the complete syntax for the configuration commands and describe what the no form of a command does.

Configuration commands also can have a default form, which returns the command settings to the default values. Most commands are disabled by default, so in such cases using the default form has the same result as using the no form of the command. However, some commands are enabled by default and

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Saving Configuration Changes

have variables set to certain default values. In these cases, the default form of the command enables the command and sets the variables to their default values. The Cisco IOS software command reference publications describe the effect of the default form of a command if the command functions differently than the no form.

Saving Configuration Changes

Use the copy system:running-config nvram:startup-config command to save your configuration changes to the startup configuration so that the changes will not be lost if the software reloads or a power outage occurs. For example:

Router# copy system:running-config nvram:startup-config Building configuration...

It might take a minute or two to save the configuration. After the configuration has been saved, the following output appears:

[OK] Router#

Using Cisco IOS Software

On most platforms, this task saves the configuration to NVRAM. On the Class A Flash file system platforms, this task saves the configuration to the location specified by the CONFIG_FILE environment variable. The CONFIG_FILE variable defaults to NVRAM.

Filtering Output from the show and more Commands

In Cisco IOS Release 12.0(1)T and later releases, you can search and filter the output of show and more commands. This functionality is useful if you need to sort through large amounts of output or if you want to exclude output that you need not see.

To use this functionality, enter a show or more command followed by the “pipe” character (|); one of the keywords begin, include, or exclude; and a regular expression on which you want to search or filter (the expression is case-sensitive):

command | {begin | include | exclude} regular-expression

The output matches certain lines of information in the configuration file. The following example illustrates how to use output modifiers with the show interface command when you want the output to include only lines in which the expression “protocol” appears:

Router# show interface | include protocol

FastEthernet0/0 is up, line protocol is up Serial4/0 is up, line protocol is up Serial4/1 is up, line protocol is up Serial4/2 is administratively down, line protocol is down Serial4/3 is administratively down, line protocol is down

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For more information on the search and filter functionality, refer to the “Using the Command-Line Interface” chapter in the Cisco IOS Configuration Fundamentals Configuration Guide.

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Using Cisco IOS Software

Identifying Supported Platforms

Cisco IOS software is packaged in feature sets consisting of software images that support specific platforms. The feature sets available for a specific platform depend on which Cisco IOS software images are included in a release. To identify the set of software images available in a specific release or to find out if a feature is available in a given Cisco IOS software image, see the following sections:

• Using Feature Navigator

• Using Software Release Notes

Using Feature Navigator

Feature Navigator is a web-based tool that enables you to quickly determine which Cisco IOS software images support a particular set of features and which features are supported in a particular Cisco IOS image.

Feature Navigator is available 24 hours a day, 7 days a week. To access Feature Navigator, you must have an account on Cisco.com. If you have forgotten or lost your account information, e-mail the Contact Database Administration group at cdbadmin@cisco.com. If you do not have an account on Cisco.com, go to http://www.cisco.com/register and follow the directions to establish an account.

To use Feature Navigator, you must have a JavaScript-enabled web browser such as Netscape 3.0 or later, or Internet Explorer 4.0 or later. Internet Explorer 4.0 always has JavaScript enabled. To enable JavaScript for Netscape 3.x or Netscape 4.x, follow the instructions provided with the web browser. For JavaScript support and enabling instructions for other browsers, check with the browser vendor.

Feature Navigator is updated when major Cisco IOS software releases and technology releases occur. You can access Feature Navigator at the following URL:

Identifying Supported Platforms

http://www.cisco.com/go/fn

Using Software Release Notes

Cisco IOS software releases include release notes that provide the following information:

• Platform support information

• Memory recommendations

• Microcode support information

• Feature set tables

• Feature descriptions

• Open and resolved severity 1 and 2 caveats for all platforms

Release notes are intended to be release-specific for the most current release, and the information provided in these documents may not be cumulative in providing information about features that first appeared in previous releases.

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Identifying Supported Platforms

Using Cisco IOS Software

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IP Overview

The Internet Protocol (IP) is a packet-based protocol used to exchange data over computer networks. IP

handles addressing, fragmentation, reassembly, and protocol demultiplexing. It is the foundation on

which all other IP protocols (collectively referred to as the IP Protocol suite) are built. A network-layer

protocol, IP contains addressing and control information that allows data packets to be routed.

The Transmission Control Protocol (TCP) is built upon the IP layer. TCP is a connection-oriented

protocol that specifies the format of data and acknowledgments used in the transfer of data. TCP also

specifies the procedures that the networking devices use to ensure that the data arrives correctly. TCP

allows multiple applications on a system to communicate concurrently because it handles all

demultiplexing of the incoming traffic among the application programs.

The Cisco implementation of IP provides most of the major services contained in the various protocol

specifications. Cisco IOS software also provides the TCP and User Datagram Protocol (UDP) services

called Echo and Discard, which are described in RFCs 862 and 863, respectively.

Cisco supports both TCP and UDP at the transport layer, for maximum flexibility in services. Cisco also

supports all standards for IP broadcasts.

This overview chapter provides a high-level description of IP. For configuration information, see the

appropriate chapter in this publication.

The Cisco IOS IP Configuration Guide has the following three parts:

• IP Addressing and Services

• IP Routing Protocols

• IP Multicast

For information on other network protocols, refer to the Cisco IOS AppleTalk and Novell IPX

Configuration Guide and Cisco IOS Apollo Domain, Banyan VINES, DECnet, ISO CLNS, and XNS

Configuration Guide.

IP Addressing and Services

IP addressing features such as Address Resolution Protocol (ARP), Next Hop Resolution Protocol

(NHRP), and Network Address Translation (NAT) are described in the “Configuring IP Addressing”

chapter. Dynamic Host Configuration Protocol (DHCP) is described in the “Configuring DHCP”

chapter.

IP services such as IP access lists, Internet Control Message Protocol (ICMP), Hot Standby Router

Protocol (HSRP), IP accounting, performance parameters, and MultiNode Balancing (MNLB)

Forwarding Agent are described in the “Configuring IP Services” chapter.

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IP Routing Protocols

Server load balancing allows a network administrator to define a virtual server to represent a group of real servers. For more information on this feature, see the “Configuring Server Load Balancing” chapter.

Mobile IP, which allows users to roam and maintain connectivity beyond their home subnet while consistently maintaining their IP address, is described in the “Configuring Mobile IP” chapter.

IP Routing Protocols

The Cisco implementation of each IP routing protocol is discussed at the beginning of the individual protocol chapters in this publication.

With any of the IP routing protocols, you must create the routing process, associate networks with the routing process, and customize the routing protocol for your particular network. You will need to perform some combination of the tasks in the respective chapters to configure one or more IP routing protocols.

Determining a Routing Process

IP Overview

Choosing a routing protocol is a complex task. When choosing a routing protocol, consider at least the following factors:

• Internetwork size and complexity

• Support for variable-length subnet masks (VLSMs). Enhanced Interior Gateway Routing Protocol

(Enhanced IGRP), Intermediate System-to-Intermediate System (IS-IS), static routes, and Open Shortest Path First (OSPF) support VLSMs.

• Internetwork traffic levels

• Security needs

• Reliability needs

• Internetwork delay characteristics

• Organizational policies

• Organizational acceptance of change

The chapters in this publication describe the configuration tasks associated with each supported routing protocol or service. This publication does not provide in-depth information on how to choose routing protocols; you must choose routing protocols that best suit your needs.

Interior and Exterior Gateway Protocols

IP routing protocols are divided into two classes: Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs). The IGPs and EGPs that Cisco supports are listed in the following sections:

IPC-2

• Interior Gateway Protocols

• Exterior Gateway Protocols

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IP Overview

Note Many routing protocol specifications refer to routers as gateways, so the word gateway often appears

as part of routing protocol names. However, a router usually is defined as a Layer 3 internetworking

device, whereas a protocol translation gateway usually is defined as a Layer 7 internetworking

device. The reader should understand that regardless of whether a routing protocol name contains the

word “gateway,” routing protocol activities occur at Layer 3 of the Open System Interconnection

(OSI) reference model.

Interior Gateway Protocols

Interior gateway protocols are used for routing networks that are under a common network

administration. All IP interior gateway protocols must be specified with a list of associated networks

before routing activities can begin. A routing process “listens” to updates from other routers on these

networks and broadcasts its own routing information on those same networks. Cisco IOS software

supports the following interior routing protocols:

• On-Demand Routing (ODR)

• Routing Information Protocol (RIP)

• Interior Gateway Routing Protocol (IGRP)

• Open Shortest Path First (OSPF)

IP Routing Protocols

• Enhanced IGRP (EIGRP)

• Integrated IS-IS

Exterior Gateway Protocols

Exterior gateway protocols are used to exchange routing information between networks that do not share

a common administration. IP Exterior Gateway Protocols require the following three sets of information

before routing can begin:

• A list of neighbor (or peer) routers with which to exchange routing information

• A list of networks to advertise as directly reachable

• The autonomous system number of the local router

The exterior gateway protocol that is supported by Cisco IOS software is Border Gateway Protocol

(BGP).

Multiprotocol BGP is an enhanced BGP that carries routing information for multiple network-layer

protocols and IP multicast routes. BGP carries two sets of routes, one set for unicast routing and one set

for multicast routing. The routes associated with multicast routing are used by Protocol Independent

Multicast (PIM) to build data distribution trees.

Multiple Routing Protocols

You can configure multiple routing protocols in a single router to connect networks that use different

routing protocols. You can, for example, run RIP on one subnetted network and IGRP on another

subnetted network, and exchange routing information between them in a controlled fashion. The

available routing protocols were not designed to interoperate, so each protocol collects different types

of information and reacts to topology changes in its own way.

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IP Multicast

IP Overview

For example, RIP uses a hop-count metric and IGRP uses a five-element vector of metric information. If routing information is being exchanged between different networks that use different routing protocols, you can use many configuration options to filter the exchange of routing information.

The Cisco IOS software can handle simultaneous operation of up to 30 dynamic IP routing processes. The combination of routing processes on a router consists of the following protocols (with the limits noted):

• Up to 30 IGRP routing processes

• Up to 30 EIGRP routing processes

• Up to 30 OSPF routing processes

• One RIP routing process

• One IS-IS process

• One BGP routing process

IP multicast routing provides an alternative to unicast and broadcast transmission. It allows a host to send packets to a subset of all hosts, known as group transmission. IP multicast runs on top of the other IP routing protocols.

In addition to IP multicast routing itself, other multicast features are available, each discussed in a separate chapter, as follows:

• Source Specific Multicast (SSM) is an extension of IP multicast where datagram traffic is forwarded

to receivers from only those multicast sources to which the receivers have explicitly joined.

• Bidirectional PIM is a variant of the PIM suite of routing protocols for IP multicast. In bidirectional

mode, datagram traffic is routed only along a bidirectional shared tree that is rooted at the rendezvous point (RP) for the multicast group.

• Multicast Source Discovery Protocol (MSDP) is a mechanism for the router to discover multicast

sources in other PIM domains.

• Pragmatic General Multicast (PGM) is a reliable multicast transport protocol for applications that

require ordered, duplicate-free, multicast data delivery from multiple sources to multiple receivers. The PGM Host feature is the Cisco implementation of the transport layer of the PGM protocol, and the PGM Router Assist feature is the Cisco implementation of the network layer of the PGM protocol.

• Unidirectional link routing (UDLR) provides a way to forward multicast packets over a physical

unidirectional interface, such as a satellite link.

• The Multicast Routing Monitor (MRM) feature is a management diagnostic tool that provides

network fault detection and isolation in a large multicast routing infrastructure. This feature is described in the “Using IP Multicast Tools” chapter.

IPC-4

• Router-Port Group Management Protocol (RGMP) is a Layer 2 protocol that enables a router to

communicate to a switch (or a networking device that is functioning as a Layer 2 switch) the multicast group for which the router would like to receive or forward traffic.

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IP Addressing and Services

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Configuring IP Addressing

This chapter describes how to configure IP addressing. For a complete description of the IP addressing

commands in this chapter, refer to the “IP Addressing Commands” chapter of the Cisco IOS IP

Command Reference, Volume 1 of 3: Addressing and Services publication. To locate documentation of

other commands that appear in this chapter, use the command reference master index, or search online.

IP Addressing Task List

A basic and required task for configuring IP is to assign IP addresses to network interfaces. Doing so

enables the interfaces and allows communication with hosts on those interfaces using IP. Associated with

this task are decisions about subnetting and masking the IP addresses.

To configure various IP addressing features, perform the tasks described in the following sections. The

task in the first section is required; the tasks in remaining sections are optional.

• Assigning IP Addresses to Network Interfaces (Required)

• Configuring Address Resolution Methods (Optional)

• Enabling IP Routing (Optional)

• Enabling IP Bridging (Optional)

• Enabling Integrated Routing and Bridging (Optional)

• Configuring a Routing Process (Optional)

• Configuring Broadcast Packet Handling (Optional)

• Configuring Network Address Translation (Optional)

• Monitoring and Maintaining IP Addressing (Optional)

At the end of this chapter, the examples in the “IP Addressing Examples” section illustrate how you

might establish IP addressing in your network.

Assigning IP Addresses to Network Interfaces

An IP address identifies a location to which IP datagrams can be sent. Some IP addresses are reserved

for special uses and cannot be used for host, subnet, or network addresses. Table 3 lists ranges of IP

addresses, and shows which addresses are reserved and which are available for use.

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Table 3 Reserved and Available IP Addresses

Class Address or Range Status

A 0.0.0.0

B 128.0.0.0 to 191.254.0.0

C 192.0.0.0

D 224.0.0.0 to 239.255.255.255 Multicast group addresses

E 240.0.0.0 to 255.255.255.254

The official description of IP addresses is found in RFC 1166, Internet Numbers.

1.0.0.0 to 126.0.0.0

127.0.0.0

191.255.0.0

192.0.1.0 to 223.255.254

223.255.255.0

255.255.255.255

Configuring IP Addressing

Reserved Available Reserved

Available Reserved

Reserved Available Reserved

Reserved Broadcast

To receive an assigned network number, contact your Internet service provider (ISP).

An interface can have one primary IP address. To assign a primary IP address and a network mask to a network interface, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip address ip-address mask

Sets a primary IP address for an interface.

A mask identifies the bits that denote the network number in an IP address. When you use the mask to subnet a network, the mask is then referred to as a subnet mask.

Note We only support network masks that use contiguous bits that are flush left against the network field.

The tasks to enable or disable additional, optional, IP addressing features are contained in the following sections:

• Assigning Multiple IP Addresses to Network Interfaces

• Enabling Use of Subnet Zero

• Disabling Classless Routing Behavior

• Enabling IP Processing on a Serial Interface

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Assigning IP Addresses to Network Interfaces

Assigning Multiple IP Addresses to Network Interfaces

Cisco IOS software supports multiple IP addresses per interface. You can specify an unlimited number

of secondary addresses. Secondary IP addresses can be used in a variety of situations. The following are

the most common applications:

• There might not be enough host addresses for a particular network segment. For example, suppose

your subnetting allows up to 254 hosts per logical subnet, but on one physical subnet you must have 300 host addresses. Using secondary IP addresses on the routers or access servers allows you to have two logical subnets using one physical subnet.

• Many older networks were built using Level 2 bridges, and were not subnetted. The judicious use of

secondary addresses can aid in the transition to a subnetted, router-based network. Routers on an older, bridged segment can easily be made aware that many subnets are on that segment.

• Two subnets of a single network might otherwise be separated by another network. You can create

a single network from subnets that are physically separated by another network by using a secondary address. In these instances, the first network is extended, or layered on top of the second network. Note that a subnet cannot appear on more than one active interface of the router at a time.

Note If any router on a network segment uses a secondary address, all other routers on that same segment

must also use a secondary address from the same network or subnet.

To assign multiple IP addresses to network interfaces, use the following command in interface

configuration mode:

Command Purpose

Router(config-if)# ip address ip-address mask secondary

Note IP routing protocols sometimes treat secondary addresses differently when sending routing updates.

Assigns multiple IP addresses to network interfaces.

See the description of IP split horizon in the “Configuring IP Enhanced IGRP,” “Configuring IGRP,”

or “Configuring RIP” chapters for details.

See the “Creating a Network from Separated Subnets Example” section at the end of this chapter for an

example of creating a network from separated subnets.

Enabling Use of Subnet Zero

Subnetting with a subnet address of 0 is illegal and strongly discouraged (as stated in RFC 791) because

of the confusion that can arise between a network and a subnet that have the same addresses. For

example, if network 131.108.0.0 is subnetted as 255.255.255.0, subnet 0 would be written as

131.108.0.0—which is identical to the network address.

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You can use the all 0s and all 1s subnet (131.108.255.0), even though it is discouraged. Configuring interfaces for the all 1s subnet is explicitly allowed. However, if you need the entire subnet space for your IP address, use the following command in global configuration mode to enable subnet 0:

Command Purpose

Router(config)# ip subnet-zero

Enables the use of subnet zero for interface addresses and routing updates.

Disabling Classless Routing Behavior

By default, classless routing behavior is enabled on the router. When classless routing is in effect, if a router receives packets destined for a subnet of a network that has no network default route, the router forwards the packet to the best supernet route.

In Figure 1, classless routing is enabled in the router. Therefore, when the host sends a packet to

128.20.4.1, instead of discarding the packet, the router forwards the packet to the best supernet route.

Figure 1 IP Classless Routing

Configuring IP Addressing

128.0.0.0/8

128.20.4.1

128.20.0.0

128.20.1.0

128.20.2.0

ip classless

128.20.3.0

128.20.4.1

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Host

If you disable classless routing, and a router receives packets destined for a subnet of a network that has no network default route, the router discards the packet. Figure 2 shows a router in network 128.20.0.0 connected to subnets 128.20.1.0, 128.20.2.0, and 128.20.3.0. Suppose the host sends a packet to

128.20.4.1. Because there is no network default route, the router discards the packet.

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Figure 2 No IP Classless Routing

128.0.0.0/8

128.20.4.1

128.20.0.0

128.20.1.0

128.20.2.0

Bit bucket

128.20.3.0

128.20.4.1

Host

To prevent the Cisco IOS software from forwarding packets destined for unrecognized subnets to the best

supernet route possible, use the following command in global configuration mode:

Command Purpose

Router(config)# no ip classless

Disables classless routing behavior.

Enabling IP Processing on a Serial Interface

You might want to enable IP processing on a serial or tunnel interface without assigning an explicit IP

address to the interface. Whenever the unnumbered interface generates a packet (for example, for a

routing update), it uses the address of the interface you specified as the source address of the IP packet.

It also uses the specified interface address in determining which routing processes are sending updates

over the unnumbered interface. Restrictions are as follows:

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• Serial interfaces using High-Level Data Link Control (HDLC), PPP, Link Access Procedure,

Balanced (LAPB), and Frame Relay encapsulations, as well as Serial Line Internet Protocol (SLIP) tunnel interfaces, can be unnumbered. Serial interfaces using Frame Relay encapsulation can also be unnumbered, but the interface must be a point-to-point subinterface. It is not possible to use the unnumbered interface feature with X.25 or Switched Multimegabit Data Service (SMDS) encapsulations.

• You cannot use the ping EXEC command to determine whether the interface is up, because the

interface has no IP address. The Simple Network Management Protocol (SNMP) can be used to remotely monitor interface status.

• You cannot netboot a runnable image over an unnumbered serial interface.

• You cannot support IP security options on an unnumbered interface.

If you are configuring Intermediate System-to-Intermediate System (IS-IS) across a serial line, you

should configure the serial interfaces as unnumbered, which allows you to conform with RFC 1195,

which states that IP addresses are not required on each interface.

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Note Using an unnumbered serial line between different major networks requires special care. If, at each

end of the link, different major networks are assigned to the interfaces you specified as unnumbered, any routing protocols running across the serial line should be configured to not advertise subnet information.

To enable IP processing on an unnumbered serial interface, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip unnumbered type number

The interface you specify must be the name of another interface in the router that has an IP address, not another unnumbered interface.

The interface you specify also must be enabled (listed as “up” in the show interfaces command display).

See the “Serial Interfaces Configuration Example” section at the end of this chapter for an example of how to configure serial interfaces.

Enables IP processing on a serial or tunnel interface without assigning an explicit IP address to the interface.

Configuring IP Addressing

Configuring Address Resolution Methods

The Cisco IP implementation allows you to control interface-specific handling of IP addresses by facilitating address resolution, name services, and other functions. The following sections describe how to configure address resolution methods:

• Establishing Address Resolution

• Mapping Host Names to IP Addresses

• Configuring HP Probe Proxy Name Requests

• Configuring the Next Hop Resolution Protocol

Establishing Address Resolution

A device in the IP can have both a local address (which uniquely identifies the device on its local segment or LAN) and a network address (which identifies the network to which the device belongs). The local address is more properly known as a data link address because it is contained in the data link layer (Layer 2 of the OSI model) part of the packet header and is read by data-link devices (bridges and all device interfaces, for example). The more technically inclined person will refer to local addresses as MAC addresses, because the MAC sublayer within the data link layer processes addresses for the layer.

To communicate with a device on Ethernet, for example, the Cisco IOS software first must determine the 48-bit MAC or local data-link address of that device. The process of determining the local data-link address from an IP address is called address resolution. The process of determining the IP address from a local data-link address is called reverse address resolution.

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Configuring Address Resolution Methods

The software uses three forms of address resolution: Address Resolution Protocol (ARP), proxy ARP,

and Probe (similar to ARP). The software also uses the Reverse Address Resolution Protocol (RARP).

ARP, proxy ARP, and RARP are defined in RFCs 826, 1027, and 903, respectively. Probe is a protocol

developed by the Hewlett-Packard Company (HP) for use on IEEE-802.3 networks.

ARP is used to associate IP addresses with media or MAC addresses. Taking an IP address as input, ARP

determines the associated media address. Once a media or MAC address is determined, the IP address

or media address association is stored in an ARP cache for rapid retrieval. Then the IP datagram is

encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP

requests and replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access

Protocol (SNAP).

RARP works the same way as ARP, except that the RARP request packet requests an IP address instead

of a local data-link address. Use of RARP requires a RARP server on the same network segment as the

router interface. RARP often is used by diskless nodes that do not know their IP addresses when they

boot. The Cisco IOS software attempts to use RARP if it does not know the IP address of an interface at

startup. Also, Cisco routers can act as RARP servers by responding to RARP requests that they are able

to answer. See the “Configure Additional File Transfer Functions” chapter in the Cisco IOS

Configuration Fundamentals Configuration Guide to learn how to configure a router as a RARP server.

The tasks required to set address resolution are contained in the following sections:

• Defining a Static ARP Cache

• Setting ARP Encapsulations

• Enabling Proxy ARP

• Configuring Local-Area Mobility

Defining a Static ARP Cache

ARP and other address resolution protocols provide a dynamic mapping between IP addresses and media

addresses. Because most hosts support dynamic address resolution, generally you need not specify static

ARP cache entries. If you must define them, you can do so globally. Performing this task installs a

permanent entry in the ARP cache. The Cisco IOS software uses this entry to translate 32-bit IP

addresses into 48-bit hardware addresses.

Optionally, you can specify that the software respond to ARP requests as if it were the owner of the

specified IP address. In case you do not want the ARP entries to be permanent, you have the option of

specifying an ARP entry timeout period when you define ARP entries.

The following two tables list the tasks to provide static mapping between IP addresses and a media

address.

Use either of the following commands in global configuration mode to specify that the software respond

to ARP requests:

Command Purpose

Router(config)# arp ip-address hardware-address type

alias

Globally associates an IP address with a media (hardware) address in the ARP cache.

Specifies that the software responds to ARP requests as if it were the owner of the specified IP address.

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Use the following command in interface configuration mode to set the length of time an ARP cache entry will stay in the cache:

Command Purpose

Router(config-if)# arp timeout seconds

Sets the length of time an ARP cache entry will stay in the cache.

To display the type of ARP being used on a particular interface and also display the ARP timeout value, use the show interfaces EXEC command. Use the show arp EXEC command to examine the contents of the ARP cache. Use the show ip arp EXEC command to show IP entries. To remove all nonstatic entries from the ARP cache, use the clear arp-cache privileged EXEC command.

Setting ARP Encapsulations

By default, standard Ethernet-style ARP encapsulation (represented by the arpa keyword) is enabled on the IP interface. You can change this encapsulation method to SNAP or HP Probe, as required by your network, to control the interface-specific handling of IP address resolution into 48-bit Ethernet hardware addresses.

Configuring IP Addressing

When you set HP Probe encapsulation, the Cisco IOS software uses the Probe protocol whenever it attempts to resolve an IEEE-802.3 or Ethernet local data-link address. The subset of Probe that performs address resolution is called Virtual Address Request and Reply. Using Probe, the router can communicate transparently with HP IEEE-802.3 hosts that use this type of data encapsulation. You must explicitly configure all interfaces for Probe that will use Probe.

To specify the ARP encapsulation type, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# arp {arpa | probe | snap}

Specifies one of three ARP encapsulation methods for a specified interface.

Enabling Proxy ARP

The Cisco IOS software uses proxy ARP (as defined in RFC 1027) to help hosts with no knowledge of routing determine the media addresses of hosts on other networks or subnets. For example, if the router receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to that host through other interfaces, then it generates a proxy ARP reply packet giving its own local data-link address. The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host. Proxy ARP is enabled by default.

To enable proxy ARP if it has been disabled, use the following command in interface configuration mode (as needed) for your network:

Command Purpose

Router(config-if)# ip proxy-arp

Cisco IOS IP Configuration Guide

Enables proxy ARP on the interface.

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Configuring Local-Area Mobility

Local-area mobility provides the ability to relocate IP hosts within a limited area without reassigning

host IP addresses and without changes to the host software. Local-area mobility is supported on Ethernet,

Token Ring, and FDDI interfaces only.

To create a mobility area with only one router, use the following commands in the interface configuration

mode:

Command Purpose

Step 1

Step 2

Router(config-if)# interface type number

Router(config-if)# ip mobile arp [timers keepalive hold-time] [access-group access-list-number | name]

To create larger mobility areas, you must first redistribute the mobile routes into your Interior Gateway

Protocol (IGP). The IGP must support host routes. You can use Enhanced Interior Gateway Routing

Protocol (IGRP), Open Shortest Path First (OSPF), IS-IS, or RIPv2. To redistribute the mobile routes

into your existing IGP configuration, use the following commands in configuration mode:

Configuring Address Resolution Methods

Enters interface configuration mode.

Enables local-area mobility.

Command Purpose

Step 1

Step 2

Router(config)# router {eigrp autonomous-system | isis [tag] | ospf process-id | rip}

Router(config)# default-metric number

Router(config)# default-metric bandwidth delay reliability loading mtu

Step 3

Router(config)# redistribute mobile

Mobile routes will always be preferred over a subnet boundary or summarized route because they are

more specific. It is important to ensure that configured or redistributed static routes do not include any

host routes for the potentially mobile hosts; otherwise, a longest match could come up with two routes

and cause ambiguity. Mobile routes will be seen as external routes to the configured routing protocol,

even within a summarization area; therefore, they will not be properly summarized by default. This is

the case even when these routes are advertised at a summarization boundary, if mobile hosts are not on

their home subnet.

Mapping Host Names to IP Addresses

Each unique IP address can have an associated host name. The Cisco IOS software maintains a cache of

host name-to-address mappings for use by the connect, telnet, and ping EXEC commands, and related

Telnet support operations. This cache speeds the process of converting names to addresses.

IP defines a naming scheme that allows a device to be identified by its location in the IP. This is a

hierarchical naming scheme that provides for domains. Domain names are pieced together with periods

(.) as the delimiting characters. For example, Cisco is a commercial organization that the IP identifies

by a com domain name, so its domain name is cisco.com. A specific device in this domain, the File

Transfer Protocol (FTP) system, for example, is identified as ftp.cisco.com.

Enters router configuration mode.

Sets default metric values.

Redistributes the mobile routes.

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To keep track of domain names, IP has defined the concept of a name server, whose job is to hold a cache (or database) of names mapped to IP addresses. To map domain names to IP addresses, you must first identify the host names, then specify a name server, and enable the Domain Naming System (DNS), the global naming scheme of the Internet that uniquely identifies network devices. These tasks are described in the following sections:

• Assigning Host Names to IP Addresses

• Specifying the Domain Name

• Specifying a Name Server

• Enabling the DNS

• Using the DNS to Discover ISO CLNS Addresses

Assigning Host Names to IP Addresses

The Cisco IOS software maintains a table of host names and their corresponding addresses, also called a host name-to-address mapping. Higher-layer protocols such as Telnet use host names to identify network devices (hosts). The router and other network devices must be able to associate host names with IP addresses to communicate with other IP devices. Host names and IP addresses can be associated with one another through static or dynamic means.

Manually assigning host names to addresses is useful when dynamic mapping is not available.

Configuring IP Addressing

To assign host names to addresses, use the following command in global configuration mode:

Command Purpose

Router(config)# ip host name [tcp-port-number] address1 [address2...address8]

Statically associates host names with IP addresses.

Specifying the Domain Name

You can specify a default domain name that the Cisco IOS software will use to complete domain name requests. You can specify either a single domain name or a list of domain names. Any IP host name that does not contain a domain name will have the domain name you specify appended to it before being added to the host table.

To specify a domain name or names, use either of the following commands in global configuration mode:

Command Purpose

Router(config)# ip domain name name

Router(config)# ip domain list name

See the “IP Domains Example” section at the end of this chapter for an example of establishing IP domains.

Defines a default domain name that the Cisco IOS software will use to complete unqualified host names.

Defines a list of default domain names to complete unqualified host names.

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Specifying a Name Server

To specify one or more hosts (up to six) that can function as a name server to supply name information

for the DNS, use the following command in global configuration mode:

Command Purpose

Router(config)# ip name-server

server-address1 [server-address2...server-address6]

Specifies one or more hosts that supply name information.

Enabling the DNS

If your network devices require connectivity with devices in networks for which you do not control name

assignment, you can assign device names that uniquely identify your devices within the entire

internetwork. The global naming scheme of the Internet, the DNS, accomplishes this task. This service

is enabled by default.

To re-enable DNS if it has been disabled, use the following command in global configuration mode:

Configuring Address Resolution Methods

Command Purpose

Router(config)# ip domain lookup

Enables DNS-based host name-to-address translation.

See the “Dynamic Lookup Example” section at the end of this chapter for an example of enabling the

DNS.

Using the DNS to Discover ISO CLNS Addresses

If your router has both IP and ISO Connectionless Network Service (ISO CLNS) enabled and you want

to use ISO CLNS network service access point (NSAP) addresses, you can use the DNS to query these

addresses, as documented in RFC 1348. This feature is enabled by default.

To disable DNS queries for ISO CLNS addresses, use the following command in global configuration

mode:

Command Purpose

Router(config)# no ip domain-lookup nsap

Disables DNS queries for ISO CLNS addresses.

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Configuring HP Probe Proxy Name Requests

HP Probe Proxy support allows the Cisco IOS software to respond to HP Probe Proxy name requests. These requests are typically used at sites that have HP equipment and are already using HP Probe Proxy. Tasks associated with HP Probe Proxy are shown in the following two tables.

To configure HP Probe Proxy, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip probe proxy

To configure HP Probe Proxy, use the following command in global configuration mode:

Command Purpose

Router(config)# ip hp-host hostname ip-address

Allows the Cisco IOS software to respond to HP Probe Proxy name requests.

Enters the host name of an HP host (for which the router is acting as a proxy) into the host table.

Configuring IP Addressing

See the “HP Hosts on a Network Segment Example” section at the end of this chapter for an example of configuring HP hosts on a network segment.

Configuring the Next Hop Resolution Protocol

Routers, access servers, and hosts can use Next Hop Resolution Protocol (NHRP) to discover the addresses of other routers and hosts connected to a nonbroadcast multiaccess (NBMA) network. Partially meshed NBMA networks are typically configured with multiple logical networks to provide full network layer connectivity. In such configurations, packets might make several hops over the NBMA network before arriving at the exit router (the router nearest the destination network). In addition, such NBMA networks (whether partially or fully meshed) typically require tedious static configurations. These static configurations provide the mapping between network layer addresses (such as IP) and NBMA addresses (such as E.164 addresses for SMDS).

NHRP provides an ARP-like solution that alleviates these NBMA network problems. With NHRP, systems attached to an NBMA network dynamically learn the NBMA address of the other systems that are part of that network, allowing these systems to directly communicate without requiring traffic to use an intermediate hop.

The NBMA network is considered nonbroadcast either because it technically does not support broadcasting (for example, an X.25 network) or because broadcasting is too expensive (for example, an SMDS broadcast group that would otherwise be too large).

The Cisco Implementation of NHRP

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The Cisco implementation of NHRP supports the IETF draft version 11 of NBMA Next Hop Resolution Protocol (NHRP).

The Cisco implementation of NHRP supports IP Version 4, Internet Packet Exchange (IPX) network layers, and, at the link layer, ATM, Ethernet, SMDS, and multipoint tunnel networks. Although NHRP is available on Ethernet, NHRP need not be implemented over Ethernet media because Ethernet is capable of broadcasting. Ethernet support is unnecessary (and not provided) for IPX.

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Configuring Address Resolution Methods

Figure 3 illustrates four routers connected to an NBMA network. Within the network are ATM or SMDS

switches necessary for the routers to communicate with each other. Assume that the switches have virtual

circuit (VC) connections represented by hops 1, 2, and 3 of the figure. When Router A attempts to

forward an IP packet from the source host to the destination host, NHRP is triggered. On behalf of the

source host, Router A sends an NHRP request packet encapsulated in an IP packet, which takes three

hops across the network to reach Router D, connected to the destination host. After receiving a positive

NHRP reply, Router D is determined to be the “NBMA next hop,” and Router A sends subsequent IP

packets for the destination to Router D in one hop.

Figure 3 Next Hop Resolution Protocol

Destination host

NBMA next hop

Router D

Hop 3

Router C

NBMA network

Router A

Subsequent

IP packets

IP NHRP

Hop 1

Source host

Hop 2

Router B

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With NHRP, once the NBMA next hop is determined, the source either starts sending data packets to the

destination (in a connectionless NBMA network such as SMDS) or establishes a virtual circuit VC

connection to the destination with the desired bandwidth and quality of service (QoS) characteristics (in

a connection-oriented NBMA network such as ATM).

Other address resolution methods can be used while NHRP is deployed. IP hosts that rely upon the

Logical IP Subnet (LIS) model might require ARP servers and services over NBMA networks, and

deployed hosts might not implement NHRP, but might continue to support ARP variations. NHRP is

designed to eliminate the suboptimal routing that results from the LIS model, and can be deployed with

existing ARP services without interfering with them.

NHRP is used to facilitate building a Virtual Private Network (VPN). In this context, a VPN consists of

a virtual Layer 3 network that is built on top of an actual Layer 3 network. The topology you use over

the VPN is largely independent of the underlying network, and the protocols you run over it are

completely independent of it.

Connected to the NBMA network are one or more stations that implement NHRP, and are known as Next

Hop Servers. All routers running Cisco IOS Release 10.3 or later releases can implement NHRP and,

thus, can act as Next Hop Servers.

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Each Next Hop Server serves a set of destination hosts, which might be directly connected to the NBMA network. Next Hop Servers cooperatively resolve the NBMA next hop addresses within their NBMA network. Next Hop Servers typically also participate in protocols used to disseminate routing information across (and beyond the boundaries of) the NBMA network, and might support ARP service.

A Next Hop Server maintains a “next hop resolution” cache, which is a table of network layer address to NBMA address mappings. The table is created from information gleaned from NHRP register packets extracted from NHRP request or reply packets that traverse the Next Hop Server as they are forwarded, or through other means such as ARP and preconfigured tables.

Protocol Operation

NHRP requests traverse one or more hops within an NBMA subnetwork before reaching the station that is expected to generate a response. Each station (including the source station) chooses a neighboring Next Hop Server to forward the request to. The Next Hop Server selection procedure typically involves performing a routing decision based upon the network layer destination address of the NHRP request. Ignoring error situations, the NHRP request eventually arrives at a station that generates an NHRP reply. This responding station either serves the destination, is the destination itself, or is a client that specified it should receive NHRP requests when it registered with its server. The responding station generates a reply using the source address from within the NHRP packet to determine where the reply should be sent.

Configuring IP Addressing

NHRP Configuration Task List

To configure NHRP, perform the tasks described in the following sections. The tasks in the first section are required; the tasks in the remaining sections are optional.

• Enabling NHRP on an Interface (Required)

• Configuring a Static IP-to-NBMA Address Mapping for a Station (Optional)

• Statically Configuring a Next Hop Server (Optional)

• Configuring NHRP Authentication (Optional)

• Controlling the Triggering of NHRP (Optional)

• Triggering NHRP Based on Traffic Thresholds (Optional)

• Controlling the NHRP Packet Rate (Optional)

• Suppressing Forward and Reverse Record Options (Optional)

• Specifying the NHRP Responder Address (Optional)

• Changing the Time Period NBMA Addresses Are Advertised as Valid (Optional)

• Configuring a GRE Tunnel for Multipoint Operation (Optional)

• Configuring NHRP Server-Only Mode (Optional)

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Enabling NHRP on an Interface

To enable NHRP for an interface on a router, use the following command in interface configuration

mode. In general, all NHRP stations within a logical NBMA network must be configured with the same

network identifier.

Command Purpose

Router(config-if)# ip nhrp network-id number

Enables NHRP on an interface.

See the “Logical NBMA Example” section and the “NHRP over ATM Example” section at the end of

this chapter for examples of enabling NHRP.

Configuring a Static IP-to-NBMA Address Mapping for a Station

To participate in NHRP, a station connected to an NBMA network should be configured with the IP and

NBMA addresses of its Next Hop Servers. The format of the NBMA address depends on the medium

you are using. For example, ATM uses an NSAP address, Ethernet uses a MAC address, and SMDS uses

an E.164 address.

These Next Hop Servers may also be the default or peer routers of the station, so their addresses can be

obtained from the network layer forwarding table of the station.

If the station is attached to several link layer networks (including logical NBMA networks), the station

should also be configured to receive routing information from its Next Hop Servers and peer routers so

that it can determine which IP networks are reachable through which link layer networks.

Configuring Address Resolution Methods

To configure static IP-to-NBMA address mapping on a station (host or router), use the following

command in interface configuration mode:

Command Purpose

Router(config-if)# ip nhrp map ip-address nbma-address

Configures static IP-to-NBMA address mapping.

Statically Configuring a Next Hop Server

A Next Hop Server normally uses the network layer forwarding table to determine where to forward

NHRP packets, and to find the egress point from an NBMA network. A Next Hop Server may alternately

be statically configured with a set of IP address prefixes that correspond to the IP addresses of the

stations it serves, and their logical NBMA network identifiers.

To statically configure a Next Hop Server, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip nhrp nhs nhs-address [net-address [netmask]]

To configure multiple networks that the Next Hop Server serves, repeat the ip nhrp nhs command with

the same Next Hop Server address, but different IP network addresses. To configure additional Next Hop

Servers, repeat the ip nhrp nhs command.

Statically configures a Next Hop Server.

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Configuring NHRP Authentication

Configuring an authentication string ensures that only routers configured with the same string can communicate using NHRP. Therefore, if the authentication scheme is to be used, the same string must be configured in all devices configured for NHRP on a fabric. To specify the authentication string for NHRP on an interface, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip nhrp authentication string

Specifies an authentication string.

Controlling the Triggering of NHRP

On any platform, there are two ways to control when NHRP is triggered. These methods are described in the following sections:

• Triggering NHRP by IP Packets

• Triggering NHRP on a per-Destination Basis

Configuring IP Addressing

Triggering NHRP by IP Packets

You can specify an IP access list that is used to decide which IP packets can trigger the sending of NHRP requests. By default, all non-NHRP packets trigger NHRP requests. To limit which IP packets trigger NHRP requests, define an access list and then apply it to the interface.

To define an access list, use the following commands in global configuration mode as needed:

Command Purpose

Router(config)# access-list access-list-number {deny | permit} source [source-wildcard]

Router(config)# access-list access-list-number {deny | permit}

protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [established] [log]

Defines a standard IP access list.

Defines an extended IP access list.

To apply the IP access list to the interface, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip nhrp interest access-list-number

Specifies an IP access list that controls NHRP requests.

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Triggering NHRP on a per-Destination Basis

By default, when the software attempts to send a data packet to a destination for which it has determined

that NHRP can be used, it sends an NHRP request for that destination. To configure the system to wait

until a specified number of data packets have been sent to a particular destination before NHRP is

attempted, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip nhrp use usage-count

Specifies how many data packets are sent to a destination before NHRP is attempted.

Triggering NHRP Based on Traffic Thresholds

NHRP can run on Cisco Express Forwarding (CEF) platforms when NHRP runs with BGP over ATM

media. You can configure NHRP to initiate switched virtual circuits (SVCs) once a configured traffic

rate is reached. Similarly, SVCs can be torn down when traffic falls to another configured rate.

Prior to Cisco IOS Release 12.0, a single packet could trigger an SVC. Now you can configure the traffic

rate that must be reached before NHRP sets up or tears down an SVC. Because SVCs are created only

for burst traffic, you can conserve resources.

Configuring Address Resolution Methods

Restrictions

Prerequisites

Cisco IOS releases prior to Release 12.0 implemented NHRP draft version 4. Cisco IOS Release 12.0

and later implements NHRP draft version 11. These versions are not compatible. Therefore, all routers

running NHRP in a network must run the same version of NHRP in order to communicate with each

other. All routers must run Cisco IOS Release 12.0 and later, or all routers must run a release prior to

Release 12.0, but not a combination of the two.

Additional restrictions:

• They work on CEF platforms only.

• They work on ATM media only.

• BGP must be configured in the network where these enhancements are running.

Before you configure the feature whereby NHRP initiation is based on traffic rate, the following

conditions must exist in the router:

• ATM must be configured.

• CEF switching or distributed CEF (dCEF) switching must be enabled.

• BGP must be configured on all routers in the network.

If you have CEF switching or dCEF switching and you want NHRP to work (whether with default values

or changed values), the ip cef accounting non-recursive command must be configured.

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NHRP Configuration Task List

To configure the NHRP triggering and teardown of SVCs based on traffic rate, perform the tasks described in the following sections. The tasks in the first section are required, the tasks in the remaining section are optional.

• Changing the Rate for Triggering SVCs (Required)

• Applying the Rates to Specific Destinations (Optional)

Changing the Rate for Triggering SVCs

When NHRP runs with BGP over ATM media, there is an additional way to control the triggering of NHRP packets. This method consists of SVCs being initiated based on the input traffic rate to a given BGP next hop.

When BGP discovers a BGP next hop and enters this BGP route into the routing table, an NHRP request is sent to the BGP next hop. When an NHRP reply is received, a subsequent route is put in the NHRP cache that directly corresponds to the BGP next hop.

A new NHRP request is sent to the same BGP next hop to repopulate the NHRP cache. When an NHRP cache entry is generated, a subsequent ATM map statement to the same BGP next hop is also created.

Configuring IP Addressing

Aggregate traffic to each BGP next hop is measured and monitored. Once the aggregate traffic has met or exceeded the configured trigger rate, NHRP creates an ATM SVC and sends traffic directly to that destination router. The router tears down the SVC to the specified destination(s) when the aggregate traffic rate falls to or below the configured teardown rate.

By default, NHRP will set up an SVC for a destination when aggregate traffic for that destination is more than 1 kbps over a running average of 30 seconds. Similarly, NHRP will tear down the SVC when the traffic for that destination drops to 0 kbps over a running average of 30 seconds. There are several ways to change the rate at which SVC set or teardown occurs. You can change the number of kbps thresholds, or the load interval, or both.

To change the number of kbps at which NHRP sets up or tears down the SVC to this destination, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip nhrp trigger-svc trigger-threshold teardown-threshold

Changes the point at which NHRP sets up or tears down SVCs.

You can change the sampling time period; that is, you can change the length of time over which the average trigger rate or teardown rate is calculated. By default, the period is 30 seconds; the range is from 30 to 300 seconds in 30-second increments. This period is for calculations of aggregate traffic rate internal to Cisco IOS software only, and it represents a worst case time period for taking action. In some cases, the software will act sooner, depending on the ramp-up and fall-off rate of the traffic.

To change the sampling time period during which threshold rates are averaged, use the following command in global configuration mode:

Command Purpose

Router(config)# ip cef traffic-statistics [load-interval seconds]

Changes the length of time in a sampling period during which trigger and teardown thresholds are averaged.

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If your Cisco hardware has a Virtual Interface Processor, version 2 adapter, you must perform the

following task to change the sampling time. By default, the port adapter sends the traffic statistics to the

Route Processor every 10 seconds. If you are using NHRP in dCEF switching mode, you must change

this update rate to 5 seconds. To do so, use the following command in global configuration mode:

Command Purpose

Router(config)# ip cef traffic-statistics [update-rate seconds]

Changes the rate at which the port adapter sends traffic statistics to the RP.

Applying the Rates to Specific Destinations

By default, all destinations are measured and monitored for NHRP triggering. However, you can choose

to impose the triggering and teardown rates on certain destinations. To do so, use the following

commands beginning in global configuration mode:

Command Purpose

Step 1

Router(config)# access-list access-list-number {deny | permit} source [source-wildcard]

Defines a standard or extended IP access list.

Configuring Address Resolution Methods

access-list access-list-number {deny | permit} protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos]

[log]

Step 2

Step 3

Router(config)# interface type number

Router(interface config)# ip nhrp interest access-list

Enters interface configuration mode.

Assigns the access list created in Step 1 that determines which destinations are included in or excluded from the SVC triggering.

For an example of setting the load interval, see the section “Changing the Rate for Triggering SVCs

Example” at the end of this chapter. For an example of applying rates to destinations, see the section

“Applying NHRP Rates to Specific Destinations Example” at the end of this chapter.

Controlling the NHRP Packet Rate

By default, the maximum rate at which the software sends NHRP packets is 5 packets per 10 seconds.

The software maintains a per- interface quota of NHRP packets (whether generated locally or forwarded)

that can be sent. To change this maximum rate, use the following command in interface configuration

mode:

Command Purpose

Router(config-if)# ip nhrp max-send pkt-count every interval

Changes the NHRP packet rate per interface.

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Suppressing Forward and Reverse Record Options

To dynamically detect link layer filtering in NBMA networks (for example, SMDS address screens), and to provide loop detection and diagnostic capabilities, NHRP incorporates a Route Record in request and reply packets. The Route Record options contain the network (and link layer) addresses of all intermediate Next Hop Servers between source and destination (in the forward direction) and between destination and source (in the reverse direction).

By default, Forward Record options and Reverse Record options are included in NHRP request and reply packets. To suppress the use of these options, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# no ip nhrp record

Suppresses Forward and Reverse Record options.

Specifying the NHRP Responder Address

If an NHRP requester wants to know which Next Hop Server generates an NHRP reply packet, it can request that information by including the responder address option in its NHRP request packet. The Next Hop Server that generates the NHRP reply packet then complies by inserting its own IP address in the NHRP reply. The Next Hop Server uses the primary IP address of the specified interface.

To specify which interface the Next Hop Server uses for the NHRP responder IP address, use the following command in interface configuration mode:

Configuring IP Addressing

Command Purpose

Router(config-if)# ip nhrp responder type number

Specifies which interface the Next Hop Server uses to determine the NHRP responder address.

If an NHRP reply packet being forwarded by a Next Hop Server contains the IP address of that server, the Next Hop Server generates an error indication of type “NHRP Loop Detected” and discards the reply.

Changing the Time Period NBMA Addresses Are Advertised as Valid

You can change the length of time that NBMA addresses are advertised as valid in positive NHRP responses. In this context, advertised means how long the Cisco IOS software tells other routers to keep the addresses it is providing in NHRP responses. The default length of time is 7200 seconds (2 hours). To change the length of time, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip nhrp holdtime seconds

Specifies the number of seconds that NBMA addresses are advertised as valid in positive NHRP responses.

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Configuring a GRE Tunnel for Multipoint Operation

You can enable a generic routing encapsulation (GRE) tunnel to operate in multipoint fashion. A tunnel

network of multipoint tunnel interfaces can be thought of as an NBMA network. To configure the tunnel,

use the following commands in interface configuration mode:

Command Purpose

Step 1

Step 2

Router(config-if)# tunnel mode gre ip multipoint

Router(config-if)# tunnel key key-number

The tunnel key should correspond to the NHRP network identifier specified in the ip nhrp network-id

interface configuration command. See the “NHRP on a Multipoint Tunnel Example” section at the end

of this chapter for an example of NHRP configured on a multipoint tunnel.

Configuring NHRP Server-Only Mode

You can configure an interface so that it cannot initiate NHRP requests or set up NHRP shortcut SVCs

but can only respond to NHRP requests. Configure NHRP server-only mode on routers you do not want

placing NHRP requests.

If an interface is placed in NHRP server-only mode, you have the option to specify the non-caching

keyword. In this case, NHRP does not store information in the NHRP cache, such as NHRP responses

that could be used again. To save memory, the non caching option is generally used on a router located

between two other routers.

To configure NHRP server-only mode, use the following command in interface configuration mode:

Enabling IP Routing

Enables a GRE tunnel to be used in multipoint fashion.

Configures a tunnel identification key.

Command Purpose

Router(config-if)# ip nhrp server-only [non-caching]

Configures NHRP server-only mode.

Enabling IP Routing

IP routing is automatically enabled in the Cisco IOS software. If you choose to set up the router to bridge

rather than route IP datagrams, you must disable IP routing. To re-enable IP routing if it has been

disabled, use the following command in global configuration mode:

Command Purpose

Router(config)# ip routing

When IP routing is disabled, the router will act as an IP end host for IP packets destined for or sourced

by it, whether or not bridging is enabled for those IP packets not destined for the device. To re-enable IP

routing, use the ip routing command.

Enables IP routing.

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Enabling IP Routing

Routing Assistance When IP Routing Is Disabled

The Cisco IOS software provides three methods by which the router can learn about routes to other networks when IP routing is disabled and the device is acting as an IP host. These methods are described in the sections that follow:

• Proxy ARP

• Default Gateway (also known as default router)

• ICMP Router Discovery Protocol

When IP routing is disabled, the default gateway feature and the router discovery client are enabled, and proxy ARP is disabled. When IP routing is enabled, the default gateway feature is disabled and you can configure proxy ARP and the router discovery servers.

Proxy ARP

The most common method of learning about other routes is by using proxy ARP. Proxy ARP, defined in RFC 1027, enables an Ethernet host with no knowledge of routing to communicate with hosts on other networks or subnets. Such a host assumes that all hosts are on the same local Ethernet, and that it can use ARP to determine their hardware addresses.

Under proxy ARP, if a device receives an ARP request for a host that is not on the same network as the ARP request sender, the Cisco IOS software evaluates whether it has the best route to that host. If it does, the device sends an ARP reply packet giving its own Ethernet hardware address. The host that sent the ARP request then sends its packets to the device, which forwards them to the intended host. The software treats all networks as if they are local and performs ARP requests for every IP address. This feature is enabled by default. If it has been disabled, see the section “Enabling Proxy ARP” earlier in this chapter.

Configuring IP Addressing

Proxy ARP works as long as other routers support it. Many other routers, especially those loaded with host-based routing software, do not support it.

Default Gateway

Another method for locating routes is to define a default router (or gateway). The Cisco IOS software sends all nonlocal packets to this router, which either routes them appropriately or sends an IP Control Message Protocol (ICMP) redirect message back, telling the router of a better route. The ICMP redirect message indicates which local router the host should use. The software caches the redirect messages and routes each packet thereafter as efficiently as possible. The limitations of this method are that there is no means of detecting when the default router has gone down or is unavailable, and there is no method of picking another device if one of these events should occur.

To set up a default gateway for a host, use the following command in global configuration mode:

Command Purpose

Router(config)# ip default-gateway ip-address

To display the address of the default gateway, use the show ip redirects EXEC command.

Sets up a default gateway (router).

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ICMP Router Discovery Protocol

The Cisco IOS software provides a third method, called router discovery, by which the router

dynamically learns about routes to other networks using the ICMP Router Discovery Protocol IRDP).

IRDP allows hosts to locate routers. When the device operates as a client, router discovery packets are

generated. When the device operates as a host, router discovery packets are received. The Cisco IRDP

implementation fully conforms to the router discovery protocol outlined in RFC 1256.

The software is also capable of wire-tapping Routing Information Protocol (RIP) and Interior Gateway

Routing Protocol (IGRP) routing updates and inferring the location of routers from those updates. The

client/server implementation of router discovery does not actually examine or store the full routing

tables sent by routing devices, it merely keeps track of which systems are sending such data.

You can configure the four protocols in any combination. We recommend that you use IRDP when

possible because it allows each router to specify both a priority and the time after which a device should

be assumed down if no further packets are received. Devices discovered using IGRP are assigned an

arbitrary priority of 60. Devices discovered through RIP are assigned a priority of 50. For IGRP and RIP,

the software attempts to measure the time between updates, and assumes that the device is down if no

updates are received for 2.5 times that interval.

Each device discovered becomes a candidate for the default router. The list of candidates is scanned and

a new highest-priority router is selected when any of the following events occurs:

Enabling IP Routing

• When a higher-priority router is discovered (the list of routers is polled at 5-minute intervals).

• When the current default router is declared down.

• When a TCP connection is about to time out because of excessive retransmissions. In this case, the

server flushes the ARP cache and the ICMP redirect cache, and picks a new default router in an attempt to find a successful route to the destination.

Enabling IRDP Processing

Only one task for configuring IRDP routing on a specified interface is required. To enable IRDP

processing on an interface, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip irdp

Enables IRDP processing on an interface.

Changing IRDP Parameters

When you enable IRDP processing, the default parameters will apply. To optionally change any of these

IRDP parameters, use the following commands in interface configuration mode, as needed:

Command Purpose

Router(config-if)# ip irdp multicast

Sends IRDP advertisements to the all-systems multicast address (224.0.0.1) on a specified interface.

Router(config-if)# ip irdp holdtime seconds

Router(config-if)# ip irdp maxadvertinterval

seconds

Router(config-if)# ip irdp minadvertinterval

seconds

Sets the IRDP period for which advertisements are valid.

Sets the IRDP maximum interval between advertisements.

Sets the IRDP minimum interval between advertisements.

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Command Purpose

Router(config-if)# ip irdp preference number

Router(config-if)# ip irdp address address [number]

The Cisco IOS software can proxy-advertise other machines that use IRDP; however, this practice is not recommended because it is possible to advertise nonexistent machines or machines that are down.

Sets the IRDP preference level of the device.

Specifies an IRDP address and preference to proxy-advertise.

Enabling IP Bridging

To transparently bridge IP on an interface, use the following commands beginning in global configuration mode:

Command Purpose

Step 1

Step 2

Step 3

Router(config)# no ip routing

Router(config)# interface type number

Router(config-if)# bridge-group group

Disables IP routing.

Specifies an interface and enters interface configuration mode.

Adds the interface to a bridge group.

Configuring IP Addressing

Enabling Integrated Routing and Bridging

With integrated routing and bridging (IRB), you can route IP traffic between routed interfaces and bridge groups, or route IP traffic between bridge groups. Specifically, local or unroutable traffic is bridged among the bridged interfaces in the same bridge group, while routable traffic is routed to other routed interfaces or bridge groups. IRB can be used to switch packets in the following ways:

• From a bridged interface to a routed interface

• From a routed interface to a bridged interface

• Within the same bridge group

For more information about configuring integrated routing and bridging, refer to the “Configuring Transparent Bridging” chapter in the Cisco IOS Bridging and IBM Networking Configuration Guide.

Configuring a Routing Process

At this point in the configuration process, you can choose to configure one or more of the many routing protocols that are available, based on your individual network needs. Routing protocols provide topology information of an internetwork. Refer to subsequent chapters in this document for the tasks involved in configuring IP routing protocols such as BGP, On-Demand Routing (ODR), RIP, IGRP, OSPF, IP Enhanced IGRP, Integrated IS-IS, and IP multicast routing. If you want to continue to perform IP addressing tasks, continue reading the following sections.

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Configuring Broadcast Packet Handling

A broadcast is a data packet destined for all hosts on a particular physical network. Network hosts

recognize broadcasts by special addresses. Broadcasts are heavily used by some protocols, including

several important Internet protocols. Control of broadcast messages is an essential responsibility of the

IP network administrator.

The Cisco IOS software supports two kinds of broadcasting: directed broadcasting and flooding. A

directed broadcast is a packet sent to a specific network or series of networks, while a flooded broadcast

packet is sent to every network. A directed broadcast address includes the network or subnet fields.

Several early IP implementations do not use the current broadcast address standard. Instead, they use the

old standard, which calls for all 0s instead of all 1s to indicate broadcast addresses. Many of these

implementations do not recognize an all-1s broadcast address and fail to respond to the broadcast

correctly. Others forward all-1s broadcasts, which causes a serious network overload known as a

broadcast storm. Implementations that exhibit these problems include systems based on versions of

Berkeley Standard Distribution (BSD) UNIX prior to Version 4.3.

Routers provide some protection from broadcast storms by limiting their extent to the local cable.

Bridges (including intelligent bridges), because they are Layer 2 devices, forward broadcasts to all

network segments, thus propagating all broadcast storms.

The best solution to the broadcast storm problem is to use a single broadcast address scheme on a

network. Most modern IP implementations allow the network manager to set the address to be used as

the broadcast address. Many implementations, including the one in the Cisco IOS software, accept and

interpret all possible forms of broadcast addresses.

For detailed discussions of broadcast issues in general, see RFC 919, Broadcasting Internet Datagrams,

and RFC 922, Broadcasting IP Datagrams in the Presence of Subnets. The support for Internet

broadcasts generally complies with RFC 919 and RFC 922; it does not support multisubnet broadcasts

as defined in RFC 922.

Configuring Broadcast Packet Handling

The current broadcast address standard provides specific addressing schemes for forwarding broadcasts.

To enable these schemes, perform the tasks described in the following sections. The task in the first

section is required; the tasks in the remaining sections are optional.

• Enabling Directed Broadcast-to-Physical Broadcast Translation (Required)

• Forwarding UDP Broadcast Packets and Protocols (Optional)

• Establishing an IP Broadcast Address (Optional)

• Flooding IP Broadcasts (Optional)

See the “Broadcasting Examples” section at the end of this chapter for broadcasting configuration

examples.

Enabling Directed Broadcast-to-Physical Broadcast Translation

By default, IP directed broadcasts are dropped; they are not forwarded. Dropping IP directed broadcasts

makes routers less susceptible to denial-of-service attacks.

You can enable forwarding of IP directed broadcasts on an interface where the broadcast becomes a

physical broadcast. If such forwarding is enabled, only those protocols configured using the

ip forward-protocol global configuration command are forwarded.

You can specify an access list to control which broadcasts are forwarded. When an access list is

specified, only those IP packets permitted by the access list are eligible to be translated from directed

broadcasts to physical broadcasts.

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Configuring Broadcast Packet Handling

To enable forwarding of IP directed broadcasts, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip directed-broadcast [access-list-number]

Enables directed broadcast-to-physical broadcast translation on an interface.

Forwarding UDP Broadcast Packets and Protocols

Network hosts occasionally use User Datagram Protocol (UDP) broadcasts to determine address, configuration, and name information. If such a host is on a network segment that does not include a server, UDP broadcasts normally are not forwarded. You can remedy this situation by configuring the interface of your router to forward certain classes of broadcasts to a helper address. You can use more than one helper address per interface.

You can specify a UDP destination port to control which UDP services are forwarded. You can specify multiple UDP protocols. You can also specify the Network Disk (ND) protocol, which is used by older diskless Sun workstations, and you can specify the network security protocol, Software Defined Network Service (SDNS). By default, both UDP and ND forwarding are enabled if a helper address has been defined for an interface. The description for the ip forward-protocol global configuration command in the Cisco IOS IPCommand Reference, Volume 1 of 3: Addressing and Services publication lists the ports that are forwarded by default if you do not specify any UDP ports.

Configuring IP Addressing

If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry Dynamic Host Configuration Protocol (DHCP) information, which means that the Cisco IOS software is compatible with DHCP clients. (DHCP is defined in RFC 1531.)

To enable forwarding and to specify the destination address, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip helper-address address

Enables forwarding and specifies the destination address for forwarding UDP broadcast packets, such as BOOTP and DHCP.

To specify which protocols will be forwarded, use the following command in global configuration mode:

Command Purpose

Router(config)# ip forward-protocol {udp [port] | nd | sdns}

Specifies which protocols will be forwarded over which ports.

See the “Helper Addresses Example” section at the end of this chapter for an example of how to configure helper addresses.

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Establishing an IP Broadcast Address

The Cisco IOS software supports IP broadcasts on both LANs and WANs. There are several ways to

indicate an IP broadcast address. Currently, the most popular way, and the default, is an address

consisting of all 1s (255.255.255.255), although the software can be configured to generate any form of

IP broadcast address. Cisco software can receive and understand any form of IP broadcast.

To set the IP broadcast address, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip broadcast-address [ip-address]

If the router does not have nonvolatile memory, and you need to specify the broadcast address to use

before the software is configured, you must change the IP broadcast address by setting jumpers in the

processor configuration register. Setting bit 10 causes the device to use all 0s. Bit 10 interacts with bit

14, which controls the network and subnet portions of the broadcast address. Setting bit 14 causes the

device to include the network and subnet portions of its address in the broadcast address. Table 4 shows

the combined effect of setting bits 10 and 14.

Establishes a different broadcast address (other than 255.255.255.255).

Configuring Broadcast Packet Handling

Table 4 Configuration Register Settings for Broadcast Address Destination

Bit 14 Bit 10 Address (<net><host>)

Out Out <ones><ones>

Out In <zeros><zeros>

In In <net><zeros>

In Out <net><ones>

Some router platforms allow the configuration register to be set through the software; see the

“Rebooting” chapter of the Cisco IOS Configuration Fundamentals Configuration Guide for details. For

other router platforms, the configuration register must be changed through hardware; see the appropriate

hardware installation and maintenance manual for your system.

Flooding IP Broadcasts

You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion using

the database created by the bridging spanning-tree protocol. Turning on this feature also prevents loops.

In order to support this capability, the routing software must include the transparent bridging, and

bridging must be configured on each interface that is to participate in the flooding. If bridging is not

configured on an interface, it still will be able to receive broadcasts. However, the interface will never

forward broadcasts it receives, and the router will never use that interface to send broadcasts received on

a different interface.

Packets that are forwarded to a single network address using the IP helper address mechanism can be

flooded. Only one copy of the packet is sent on each network segment.

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Configuring Broadcast Packet Handling

In order to be considered for flooding, packets must meet the following criteria. (Note that these are the same conditions used to consider packet forwarding using IP helper addresses.)

• The packet must be a MAC-level broadcast.

• The packet must be an IP-level broadcast.

• The packet must be a Trivial File Transfer Protocol (TFTP), DNS, Time, NetBIOS, ND, or BOOTP

packet, or a UDP protocol specified by the ip forward-protocol udp global configuration command.

• The time-to-live (TTL) value of the packet must be at least two.

A flooded UDP datagram is given the destination address you specified with the ip broadcast-address command in the interface configuration mode on the output interface. The destination address can be set to any desired address. Thus, the destination address may change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.

After a decision has been made to send the datagram out on an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they are present on the output interface.

To use the bridging spanning-tree database to flood UDP datagrams, use the following command in global configuration mode:

Configuring IP Addressing

Command Purpose

Router(config)# ip forward-protocol spanning-tree

Uses the bridging spanning-tree database to flood UDP datagrams.

If no actual bridging is desired, you can configure a type-code bridging filter that will deny all packet types from being bridged. Refer to the “Configuring Transparent Bridging” chapter of the Cisco IOS Bridging and IBM Networking Configuration Guide for more information about using access lists to filter bridged traffic. The spanning-tree database is still available to the IP forwarding code to use for the flooding.

Speeding Up Flooding of UDP Datagrams

You can speed up flooding of UDP datagrams using the spanning-tree algorithm. Used in conjunction with the ip forward-protocol spanning-tree command in global configuration mode, this feature boosts the performance of spanning tree-based UDP flooding by a factor of about four to five times. The feature, called turbo flooding, is supported over Ethernet interfaces configured for Advanced Research Projects Agency (ARPA) encapsulated, FDDI, and HDLC-encapsulated serial interfaces. However, it is not supported on Token Ring interfaces. As long as the Token Rings and the non-HDLC serial interfaces are not part of the bridge group being used for UDP flooding, turbo flooding will behave normally.

To enable turbo flooding, use the following command in global configuration mode:

Command Purpose

Router(config)# ip forward-protocol turbo-flood

Uses the bridging spanning-tree database to speed up flooding of UDP datagrams.

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Configuring Network Address Translation

Two key problems facing the Internet are depletion of IP address space and scaling in routing. Network

Address Translation (NAT) is a feature that allows the IP network of an organization to appear from the

outside to use different IP address space than what it is actually using. Thus, NAT allows an organization

with nonglobally routable addresses to connect to the Internet by translating those addresses into

globally routable address space. NAT also allows a more graceful renumbering strategy for organizations

that are changing service providers or voluntarily renumbering into classless interdomain routing

(CIDR) blocks. NAT is also described in RFC 1631.

Beginning with Cisco IOS Release 12.1(5)T, NAT supports all H.225 and H.245 message types,

including FastConnect and Alerting as part of the H.323 version 2 specification. Any product that makes

use of these message types will be able to pass through a Cisco IOS NAT configuration without any static

configuration. Full support for NetMeeting Directory (Internet Locator Service) is also provided through

Cisco IOS NAT.

NAT Applications

NAT has several applications. Use it for the following purposes:

• You want to connect to the Internet, but not all your hosts have globally unique IP addresses. NAT

enables private IP internetworks that use nonregistered IP addresses to connect to the Internet. NAT is configured on the router at the border of a stub domain (referred to as the inside network) and a public network such as the Internet (referred to as the outside network). NAT translates the internal local addresses to globally unique IP addresses before sending packets to the outside network.

Configuring Network Address Translation

Benefits

• You must change your internal addresses. Instead of changing them, which can be a considerable

amount of work, you can translate them by using NAT.

• You want to do basic load sharing of TCP traffic. You can map a single global IP address to many

local IP addresses by using the TCP load distribution feature.

As a solution to the connectivity problem, NAT is practical only when relatively few hosts in a stub

domain communicate outside of the domain at the same time. When this is the case, only a small subset

of the IP addresses in the domain must be translated into globally unique IP addresses when outside

communication is necessary, and these addresses can be reused when no longer in use.

A significant advantage of NAT is that it can be configured without requiring changes to hosts or routers

other than those few routers on which NAT will be configured. As discussed previously, NAT may not

be practical if large numbers of hosts in the stub domain communicate outside of the domain.

Furthermore, some applications use embedded IP addresses in such a way that it is impractical for a NAT

device to translate. These applications may not work transparently or at all through a NAT device. NAT

also hides the identity of hosts, which may be an advantage or a disadvantage.

A router configured with NAT will have at least one interface to the inside and one to the outside. In a

typical environment, NAT is configured at the exit router between a stub domain and backbone. When a

packet is leaving the domain, NAT translates the locally significant source address into a globally unique

address. When a packet is entering the domain, NAT translates the globally unique destination address

into a local address. If more than one exit point exists, each NAT must have the same translation table.

If the software cannot allocate an address because it has run out of addresses, it drops the packet and

sends an ICMP host unreachable packet.

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A router configured with NAT must not advertise the local networks to the outside. However, routing information that NAT receives from the outside can be advertised in the stub domain as usual.

NAT Terminology

As mentioned previously, the term inside refers to those networks that are owned by an organization and that must be translated. Inside this domain, hosts will have addresses in the one address space, while on the outside, they will appear to have addresses in another address space when NAT is configured. The first address space is referred to as the local address space and the second is referred to as the global address space.

Similarly, outside refers to those networks to which the stub network connects, and which are generally not under the control of the organization. Hosts in outside networks can be subject to translation also, and can thus have local and global addresses.

To summarize, NAT uses the following definitions:

• Inside local address—The IP address that is assigned to a host on the inside network. The address

• Inside global address—A legitimate IP address (assigned by the NIC or service provider) that

Configuring IP Addressing

is probably not a legitimate IP address assigned by the Network Information Center (NIC) or service provider.

represents one or more inside local IP addresses to the outside world.

• Outside local address—The IP address of an outside host as it appears to the inside network. Not

necessarily a legitimate address, it was allocated from address space routable on the inside.

• Outside global address—The IP address assigned to a host on the outside network by the owner of

the host. The address was allocated from globally routable address or network space.

NAT Configuration Task List

Before configuring any NAT translation, you must know your inside local addresses and inside global addresses. To configure NAT, perform the optional tasks described in the following sections:

• Translating Inside Source Addresses (Optional)

• Overloading an Inside Global Address Optional)

• Translating Overlapping Addresses (Optional)

• Providing TCP Load Distribution (Optional)

• Changing Translation Timeouts (Optional)

• Monitoring and Maintaining NAT(Optional)

• Deploying NAT Between an IP Phone and Cisco CallManager (Optional)

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Translating Inside Source Addresses

You can translate your own IP addresses into globally unique IP addresses when communicating outside

of your network. You can configure static or dynamic inside source translation as follows:

• Static translation establishes a one-to-one mapping between your inside local address and an inside

global address. Static translation is useful when a host on the inside must be accessible by a fixed address from the outside.

• Dynamic translation establishes a mapping between an inside local address and a pool of global

addresses. An access-list or a route-map can be specified for dynamic translations. Route maps allow you to match any combination of access-list, new-hop IP address, and output interface to determine which pool to use.

Figure 4 illustrates a router that is translating a source address inside a network to a source address

outside the network.

Figure 4 NAT Inside Source Translation

Configuring Network Address Translation

1.1.1.2

1.1.1.1

Inside

1.1.1.1 1

1.1.1.1

Inside

interface

Inside Local

IP Address

1.1.1.2

1.1.1.1

Outside

NAT table

Inside Global

IP Address

2.2.2.2

Outside

interface

2.2.2.3

2.2.2.2

Internet

Host B

9.6.7.3

The following process describes inside source address translation, as shown in Figure 4:

1. The user at host 1.1.1.1 opens a connection to host B.

2. The first packet that the router receives from host 1.1.1.1 causes the router to check its NAT table:

–

If a static translation entry was configured, the router goes to Step 3.

–

If no translation entry exists, the router determines that Source-Address (SA) 1.1.1.1 must be translated dynamically, selects a legal, global address from the dynamic address pool, and creates a translation entry. This type of entry is called a simple entry.

3. The router replaces the inside local source address of host 1.1.1.1 with the global address of the

translation entry and forwards the packet.

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4. Host B receives the packet and responds to host 1.1.1.1 by using the inside global IP Destination-

Address (DA) 2.2.2.2.

5. When the router receives the packet with the inside global IP address, it performs a NAT table

lookup by using the inside global address as a key. It then translates the address to the inside local address of host 1.1.1.1 and forwards the packet to host 1.1.1.1.

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Host 1.1.1.1 receives the packet and continues the conversation. The router performs Steps 2 through 5 for each packet.

Configuring Static Translation

To configure static inside source address translation, use the following commands in global configuration mode:

Command Purpose

Step 1

Step 2

Step 3

Step 4

Step 5

Router(config)# ip nat inside source static local-ip global-ip

Router(config)# interface type number

Router(config-if)# ip nat inside

Router(config)# interface type number

Router(config-if)# ip nat outside

Configuring IP Addressing

Establishes static translation between an inside local address and an inside global address.

Specifies the inside interface and enters interface configuration mode.

Marks the interface as connected to the inside.

Specifies the outside interface and enters interface configuration mode.

Marks the interface as connected to the outside.

The previous steps are the minimum you must configure. You could also configure multiple inside and outside interfaces.

Configuring Dynamic Translation with an Access List

To configure dynamic inside source address translation with an access list, use the following commands in global configuration mode:

Command Purpose

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7

Router(config)# ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length}

Router(config)# access-list access-list-number permit source [source-wildcard]

Router(config)# ip nat inside source list

access-list-number pool name

Router(config)# interface type number

Router(config-if)# ip nat inside

Router(config)# interface type number

Router(config-if)# ip nat outside

Defines a pool of global addresses to be allocated as needed.

Defines a standard access list permitting those addresses that are to be translated.

Establishes dynamic source translation, specifying the access list defined in the prior step.

Specifies the inside interface and enters interface configuration mode.

Marks the interface as connected to the inside.

Specifies the outside interface and enters interface configuration mode.

Marks the interface as connected to the outside.

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Note The access list must permit only those addresses that are to be translated. (Remember that there is an

implicit “deny all” at the end of each access list.) An access list that is too permissive can lead to unpredictable results.

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Packets that enter the router through the inside interface and packets sourced from the router are checked against the access list for possible NAT candidates. The access list is used to specify which traffic is to be translated.

Configuring Dynamic Translation with a Route Map

To configure dynamic inside source address translation with a route map, use the following commands in global configuration mode:

Command Purpose

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7

Router(config)# ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length}

Router(config)# route-map name permit sequence

Router(config)# ip nat inside source route-map name

pool name

Router(config)# interface type number

Router(config-if)# ip nat inside

Router(config)# interface type number

Router(config-if)# ip nat outside

Configuring Network Address Translation

Defines a pool of global addresses to be allocated as needed.

Defines a route map permitting those addresses that are to be translated.

Establishes dynamic source translation, specifying the route map defined in the prior step.

Specifies the inside interface and enters interface configuration mode.

Marks the interface as connected to the inside.

Specifies the outside interface and enters interface configuration mode.

Marks the interface as connected to the outside.

See the “Dynamic Inside Source Translation Example” section at the end of this chapter for examples of dynamic inside source translation.

Overloading an Inside Global Address

You can conserve addresses in the inside global address pool by allowing the router to use one global address for many local addresses. When this overloading is configured, the router maintains enough information from higher-level protocols (for example, TCP or UDP port numbers) to translate the global address back to the correct local address. When multiple local addresses map to one global address, the TCP or UDP port numbers of each inside host distinguish between the local addresses.

Figure 5 illustrates NAT operation when one inside global address represents multiple inside local

addresses. The TCP port numbers act as differentiators.

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Figure 5 NAT Overloading Inside Global Addresses

1.1.1.2

1.1.1.1

Inside

1.1.1.1

1.1.1.1 1

NAT table

Protocol Inside Global IP

Inside Local IP

address:port

2.2.2.2 Internet

Outside Global

IP address:port

Configuring IP Addressing

2.2.2.2

Host B

2.2.2.2

9.6.7.3

Host C

6.5.4.7

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TCP TCP

1.1.1.2:1723

1.1.1.1:1024

2.2.2.2:1723

2.2.2.2:1024

6.5.4.7:23

9.6.7.3:23

The router performs the following process in overloading inside global addresses, as shown in Figure 5. Both host B and host C believe they are communicating with a single host at address 2.2.2.2. They are actually communicating with different hosts; the port number is the differentiator. In fact, many inside hosts could share the inside global IP address by using many port numbers.

1. The user at host 1.1.1.1 opens a connection to host B.

2. The first packet that the router receives from host 1.1.1.1 causes the router to check its NAT table:

–

If no translation entry exists, the router determines that address 1.1.1.1 must be translated, and sets up a translation of inside local address 1.1.1.1 to a legal global address.

–

If overloading is enabled, and another translation is active, the router reuses the global address from that translation and saves enough information to be able to translate back. This type of entry is called an extended entry.

3. The router replaces the inside local source address 1.1.1.1 with the selected global address and

forwards the packet.

4. Host B receives the packet and responds to host 1.1.1.1 by using the inside global IP address 2.2.2.2.

5. When the router receives the packet with the inside global IP address, it performs a NAT table

lookup, using the protocol, inside global address and port, and outside address and port as a key; translates the address to inside local address 1.1.1.1; and forwards the packet to host 1.1.1.1.

Host 1.1.1.1 receives the packet and continues the conversation. The router performs Steps 2 through 5 for each packet.

Step 1

Step 2

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To configure overloading of inside global addresses, use the following commands in global configuration mode:

Command Purpose

Router(config)# ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length}

Router(config)# access-list access-list-number permit source [source-wildcard]

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Defines a pool of global addresses to be allocated as needed.

Defines a standard access list.

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Command Purpose

Step 3

Step 4

Step 5

Step 6

Step 7

Router(config)# ip nat inside source list

access-list-number pool name overload

Router(config)# interface type number

Router(config-if)# ip nat inside

Router(config)# interface type number

Router(config-if)# ip nat outside

Note The access list must permit only those addresses that are to be translated. (Remember that there is an

Configuring Network Address Translation

Establishes dynamic source translation, specifying the access list defined in the prior step.

Specifies the inside interface.

Marks the interface as connected to the inside.

Specifies the outside interface.

Marks the interface as connected to the outside.

implicit “deny all” at the end of each access list.) An access list that is too permissive can lead to unpredictable results. Packets that enter the router through the inside interface and packets sourced from the router are checked against the access list for possible NAT candidates. The access list is used to specify which traffic is to be translated.

See the “Overloading Inside Global Addresses Example” section at the end of this chapter for an example of overloading inside global addresses.

Translating Overlapping Addresses

The NAT overview discusses translating IP addresses, which could occur because your IP addresses are not legal, officially assigned IP addresses. Perhaps you chose IP addresses that officially belong to another network. The case of an address used both illegally and legally is called overlapping. You can use NAT to translate inside addresses that overlap with outside addresses. Use this feature if your IP addresses in the stub network are legitimate IP addresses belonging to another network, and you want to communicate with those hosts or routers.

Figure 6 shows how NAT translates overlapping networks.

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Figure 6 NAT Translating Overlapping Addresses

1.1.1.1

Configuring IP Addressing

DNS request for host C address

SA=2.2.2.2 DA=x.x.x.x

DNS server

x.x.x.x

Internet

DNS request for host C address

SA=1.1.1.1 DA=x.x.x.x

DNS response from x.x.x.x

SA=x.x.x.x DA=1.1.1.1

1.1.1.1 message to host C SA=1.1.1.1 DA=3.3.3.3

C=3.3.3.3

Inside Local

IP Address

1.1.1.1

DNS response from x.x.x.x

SA=x.x.x.x DA=2.2.2.2

1.1.1.1 message to host C SA=2.2.2.2 DA=1.1.1.3

NAT table

Inside Global

IP Address

2.2.2.2 1.1.1.3

C=1.1.1.3

Outside Global

IP Address

Host C

1.1.1.3

Outside Local

IP Address

3.3.3.3

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The router performs the following process when translating overlapping addresses:

1. The user at host 1.1.1.1 opens a connection to host C by name, requesting a name-to-address lookup

from a DNS server.

2. The router intercepts the DNS reply and translates the returned address if there is an overlap (that

is, the resulting legal address resides illegally in the inside network). To translate the return address, the router creates a simple translation entry mapping the overlapping address 1.1.1.3 to an address from a separately configured, outside local address pool.

The router examines every DNS reply from everywhere, ensuring that the IP address is not in the stub network. If it is, the router translates the address.

3. Host 1.1.1.1 opens a connection to 3.3.3.3.

4. The router sets up translations mapping inside local and global addresses to each other, and outside

global and local addresses to each other.

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5. The router replaces the SA with the inside global address and replaces the DA with the outside

global address.

6. Host C receives the packet and continues the conversation.

7. The router does a lookup, replaces the DA with the inside local address, and replaces the SA with

the outside local address.

8. Host 1.1.1.1 receives the packet and the conversation continues, using this translation process.

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Configuring Static Translation

To configure static SA address translation, use the following commands in global configuration mode:

Command Purpose

Step 1

Step 2

Step 3

Step 4

Step 5

Router(config)# ip nat outside source static

global-ip local-ip

Router(config)# interface type number

Router(config-if)# ip nat inside

Router(config)# interface type number

Router(config-if)# ip nat outside

Configuring Dynamic Translation

To configure dynamic outside source address translation, use the following commands in global configuration mode:

Configuring Network Address Translation

Establishes static translation between an outside local address and an outside global address.

Specifies the inside interface.

Marks the interface as connected to the inside.

Specifies the outside interface.

Marks the interface as connected to the outside.

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7

Command Purpose

Router(config)# ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length}

Router(config)# access-list access-list-number permit source [source-wildcard]

Router(config)# ip nat outside source list

access-list-number pool name

Router(config)# interface type number

Router(config-if)# ip nat inside

Router(config)# interface type number

Router(config-if)# ip nat outside

Note The access list must permit only those addresses that are to be translated. (Remember that there is an

Defines a pool of local addresses to be allocated as needed.

Defines a standard access list.

Establishes dynamic outside source translation, specifying the access list defined in the prior step.

Specifies the inside interface.

Marks the interface as connected to the inside.

Specifies the outside interface.

Marks the interface as connected to the outside.

implicit “deny all” at the end of each access list.) An access list that is too permissive can lead to unpredictable results.

See the “Translating Overlapping Address Example” section at the end of this chapter for an example of translating an overlapping address.

Providing TCP Load Distribution

Another use of NAT is unrelated to Internet addresses. Your organization may have multiple hosts that must communicate with a heavily used host. Using NAT, you can establish a virtual host on the inside network that coordinates load sharing among real hosts. DAs that match an access list are replaced with

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addresses from a rotary pool. Allocation is done on a round-robin basis, and only when a new connection is opened from the outside to the inside. Non-TCP traffic is passed untranslated (unless other translations are in effect). Figure 7 illustrates this feature.

Figure 7 NAT TCP Load Distribution

Configuring IP Addressing

Inside

9.6.7.3

6.5.4.7

1.1.1.127

Protocol

TCP TCP TCP

Intranet

Inside Local IP

address:port

1.1.1.1:23

1.1.1.2:23

1.1.1.3:23

1.1.1.127

NAT table

Inside Global IP

address:port

1.1.1.127:23

1.1.1.1 3

Outside Global

IP address:port

9.6.7.5:3058

6.5.4.7:4371

9.6.7.3:3062

1.1.1.1

Real

hosts

1.1.1.2

1.1.1.3

Virtual

host

1.1.1.127

The router performs the following process when translating rotary addresses:

1. The user on host B (9.6.7.3) opens a connection to the virtual host at 1.1.1.127.

2. The router receives the connection request and creates a new translation, allocating the next real host

(1.1.1.1) for the inside local IP address.

3. The router replaces the destination address with the selected real host address and forwards the

packet.

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4. Host 1.1.1.1 receives the packet and responds.

5. The router receives the packet, performs a NAT table lookup using the inside local address and port

number, and the outside address and port number as the key. The router then translates the source address to the address of the virtual host and forwards the packet.

The next connection request will cause the router to allocate 1.1.1.2 for the inside local address.

To configure destination address rotary translation, use the following commands beginning in global configuration mode. These commands allow you to map one virtual host to many real hosts. Each new TCP session opened with the virtual host will be translated into a session with a different real host.

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Command Purpose

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7

Router(config)# ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length} type

rotary

Router(config)# access-list access-list-number permit source [source-wildcard]

Router(config)# ip nat inside destination list access-list-number pool name

Router(config)# interface type number

Router(config-if)# ip nat inside

Router(config)# interface type number

Router(config-if)# ip nat outside

Note The access list must permit only those addresses that are to be translated. (Remember that there is an

Configuring Network Address Translation

Defines a pool of addresses containing the addresses of the real hosts.

Defines an access list permitting the address of the virtual host.

Establishes dynamic inside destination translation, specifying the access list defined in the prior step.

Specifies the inside interface.

Marks the interface as connected to the inside.

Specifies the outside interface.

Marks the interface as connected to the outside.

implicit “deny all” at the end of each access list.) An access list that is too permissive can lead to unpredictable results.

See the “ping Command Example” section at the end of this chapter for an example of rotary translation.

Changing Translation Timeouts

By default, dynamic address translations time out after some period of nonuse. You can change the default values on timeouts, if necessary. When overloading is not configured, simple translation entries time out after 24 hours. To change this value, use the following command in global configuration mode:

Command Purpose

Router(config)# ip nat translation timeout seconds

If you have configured overloading, you have more control over translation entry timeout, because each entry contains more context about the traffic using it. To change timeouts on extended entries, use the following commands in global configuration mode as needed:

Command Purpose

Router(config)# ip nat translation udp-timeout seconds

Router(config)# ip nat translation dns-timeout seconds

Router(config)# ip nat translation tcp-timeout seconds

Router(config)# ip nat translation finrst-timeout seconds

Changes the timeout value for dynamic address translations that do not use overloading.

Changes the UDP timeout value from 5 minutes.

Changes the DNS timeout value from 1 minute.

Changes the TCP timeout value from 24 hours.

Changes the Finish and Reset timeout value from 1 minute.

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Command Purpose

Router(config)# ip nat translation icmp-timeout seconds

Router(config)# ip nat translation syn-timeout seconds

Changes the ICMP timeout value from 1 minute.

Changes the Synchronous (SYN) timeout value from 1 minute.

Monitoring and Maintaining NAT

By default, dynamic address translations will time out from the NAT translation table at some point. To clear the entries before the timeout, use the following commands in EXEC mode as needed:

Command Purpose

Router# clear ip nat translation *

Router# clear ip nat translation inside global-ip local-ip [outside local-ip global-ip]

Router# clear ip nat translation outside local-ip global-ip

Router# clear ip nat translation protocol inside global-ip

global-port local-ip local-port [outside local-ip local-port global-ip global-port]

Clears all dynamic address translation entries from the NAT translation table.

Clears a simple dynamic translation entry containing an inside translation, or both inside and outside translation.

Clears a simple dynamic translation entry containing an outside translation.

Clears an extended dynamic translation entry.

Configuring IP Addressing

To display translation information, use either of the following commands in EXEC mode:

Command Purpose

Router# show ip nat translations [verbose]

Router# show ip nat statistics

Displays active translations.

Displays translation statistics.

Deploying NAT Between an IP Phone and Cisco CallManager

Cisco IP phones use the Selsius Skinny Station Protocol to connect with and register to the Cisco CallManager (CCM). Messages flow back and forth that include IP address and port information used to identify other IP phone users with which a call can be placed.

To be able to deploy Cisco IOS NAT between the IP phone and CCM in a scalable environment, NAT needs to be able to detect the Selsius Skinny Station Protocol and understand the information passed within the messages.

When an IP phone attempts to connect to the CCM and it matches the configured NAT translation rules, NAT will translate the original source IP address and replace it with one from the configured pool. This new address will be reflected in the CCM and be visible to other IP phone users.

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To specify a port other than the default port, use the following command in global configuration mode:

Command Purpose

Router(config)# ip nat service skinny tcp port

number

Displays port number on which the CCM is listening for skinny messages.

Monitoring and Maintaining IP Addressing

To monitor and maintain your network, perform the tasks described in the following sections. The tasks in the first section are required; the tasks in the remaining sections are optional.

• Clearing Caches, Tables, and Databases (Required)

• Specifying the Format of Network Masks (Optional)

• Displaying System and Network Statistics (Optional)

• Monitoring and Maintaining NHRP (Optional)

Monitoring and Maintaining IP Addressing

Clearing Caches, Tables, and Databases

You can remove all contents of a particular cache, table, or database. Clearing a cache, table, or database can become necessary when the contents of the particular structure have become or are suspected to be invalid.

To clear caches, tables, and databases, use the following commands in EXEC mode, as needed:

Command Purpose

Router# clear arp-cache

Router# clear host {name | *}

Router# clear ip route {network [mask] | *}

Clears the IP ARP cache and the fast-switching cache.

Removes one or all entries from the host name and address cache.

Removes one or more routes from the IP routing table.

Specifying the Format of Network Masks

IP uses a 32-bit mask, called a netmask, that indicates which address bits belong to the network and subnetwork fields, and which bits belong to the host field. This is called a netmask. By default, show commands display an IP address and then its netmask in dotted decimal notation. For example, a subnet would be displayed as 131.108.11.55 255.255.255.0.

You might find it more convenient to display the network mask in hexadecimal format or bit count format instead. The hexadecimal format is commonly used on UNIX systems. The previous example would be displayed as 131.108.11.55 0XFFFFFF00.

The bit count format for displaying network masks is to append a slash (/) and the total number of bits in the netmask to the address itself. The previous example would be displayed as 131.108.11.55/24.

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To specify the format in which netmasks appear for the current session, use the following command in EXEC mode:

Command Purpose

Router# term ip netmask-format {bitcount | decimal | hexadecimal}

Specifies the format of network masks for the current session.

To configure the format in which netmasks appear for an individual line, use the following command in line configuration mode:

Command Purpose

Router(config-line)# ip netmask-format {bitcount | decimal | hexadecimal}

Configures the format of network masks for a line.

Displaying System and Network Statistics

Configuring IP Addressing

You can display specific statistics such as the contents of IP routing tables, caches, and databases. The resulting information can be used to determine resource utilization and to solve network problems. You also can display information about node reachability and discover the routing path that the packets of your device are taking through the network.

These tasks are summarized in the table that follows. See the “IP Addressing Commands” chapter in the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services publication for details about the commands listed in these tasks. Use the following commands in privileged EXEC mode to display specific statistics, as needed:

Command Purpose

Router# show arp

Router# show hosts

Displays the entries in the ARP table.

Displays the default domain name, style of lookup service, the name server hosts, and the cached list of host names and addresses.

Router# show ip aliases

Router# show ip arp

Router# show ip interface [type number]

Router# show ip irdp

Router# show ip masks address

Displays IP addresses mapped to TCP ports (aliases).

Displays the IP ARP cache.

Displays the usability status of interfaces.

Displays IRDP values.

Displays the masks used for network addresses and the number of subnets using each mask.

Router# show ip redirects

Router# show ip route [address [mask] [longer-prefixes]] | [protocol [process-id]]

Router# show ip route summary

Router# ping [protocol] {host | address}

Displays the address of a default gateway.

Displays the current state of the routing table.

Displays the current state of the routing table in summary form.

Tests network node reachability (privileged mode).

Tests network node reachability using a simple ping facility (user mode).

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Command Purpose

Router# trace [destination]

Router# trace ip destination

Traces packet routes through the network (privileged mode).

Traces packet routes through the network (user mode).

See the “ping Command Example” section at the end of this chapter for an example of pinging.

Monitoring and Maintaining NHRP

To monitor the NHRP cache or traffic, use either of the following commands in EXEC mode:

Command Purpose

Router# show ip nhrp [dynamic | static] [type number]

Router# show ip nhrp traffic

Displays the IP NHRP cache, optionally limited to dynamic or static cache entries for a specific interface.

Displays NHRP traffic statistics.

IP Addressing Examples

The NHRP cache can contain static entries caused by statically configured addresses and dynamic entries caused by the Cisco IOS software learning addresses from NHRP packets. To clear static entries, use the no ip nhrp map command in interface configuration mode. To clear the NHRP cache of dynamic entries, use the following command in EXEC mode:

Command Purpose

Router# clear ip nhrp

Clears the IP NHRP cache of dynamic entries.

In a dual hub Dynamic Multipoint VPN (DMVPN) environment, when using the clear ip nhrp command on the hub, you may see the following error message on the spokes:

%NHRP-3-PAKERROR: Receive Error Indication for our Error Indication, code: protocol generic error(7), offset: 0, data: 00 01 08 00 00 00 00 00 00 FF 00 44 5F F6 00 34

This is only an informational message generated as a part of the NHRP purge notification processing and will not cause any other issues.

IP Addressing Examples

The following sections provide IP configuration examples:

• Creating a Network from Separated Subnets Example

• Serial Interfaces Configuration Example

• IP Domains Example

• Dynamic Lookup Example

• HP Hosts on a Network Segment Example

• Logical NBMA Example

• NHRP over ATM Example

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• Changing the Rate for Triggering SVCs Example

• Applying NHRP Rates to Specific Destinations Example

• NHRP on a Multipoint Tunnel Example

• Broadcasting Examples

• NAT Configuration Examples

• ping Command Example

Creating a Network from Separated Subnets Example

In the following example, subnets 1 and 2 of network 131.108.0.0 are separated by a backbone, as shown in Figure 8. The two networks are brought into the same logical network through the use of secondary addresses.

Figure 8 Creating a Network from Separated Subnets

Network 192.5.10.0

Subnet 172.16.3.0

Configuring IP Addressing

Router B

Subnet 172.16.1.0 Subnet 172.16.2.0

Router A

Router C

Router D

S1016a

The following examples show the configurations for routers B and C:

Router B Configuration

interface ethernet 2

ip address 192.5.10.1 255.255.255.0 ip address 131.108.3.1 255.255.255.0 secondary

Router C Configuration

interface ethernet 1

ip address 192.5.10.2 255.255.255.0 ip address 131.108.3.2 255.255.255.0 secondary

Serial Interfaces Configuration Example

In the following example, the second serial interface (serial 1) is given the address of Ethernet interface

0. The serial interface is unnumbered.

interface ethernet 0

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ip address 145.22.4.67 255.255.255.0

interface serial 1

ip unnumbered ethernet 0

IP Domains Example

The following example establishes a domain list with several alternate domain names:

ip domain list csi.com ip domain list telecomprog.edu ip domain-list merit.edu

Dynamic Lookup Example

A cache of host name-to-address mappings is used by connect, telnet, ping, trace, write net, and configure net EXEC commands to speed the process of converting names to addresses. The commands

used in this example specify the form of dynamic name lookup to be used. Static name lookup also can be configured.

IP Addressing Examples

The following example configures the host name-to-address mapping process. IP DNS-based translation is specified, the addresses of the name servers are specified, and the default domain name is given.

! IP Domain Name System (DNS)-based host name-to-address translation is enabled ip domain lookup ! Specifies host 131.108.1.111 as the primary name server and host 131.108.1.2 ! as the secondary server ip name-server 131.108.1.111 131.108.1.2 ! Defines cisco.com as the default domain name the router uses to complete ! unqualified host names ip domain name cisco.com

HP Hosts on a Network Segment Example

The following example has a network segment with HP devices on it. The commands in this example customize the first Ethernet port to respond to Probe name requests for the host name, and to use Probe and ARP.

ip hp-host bl4zip 131.24.6.27 interface ethernet 0 arp probe ip probe proxy

Logical NBMA Example

A logical NBMA network is considered the group of interfaces and hosts participating in NHRP and having the same network identifier. Figure 9 illustrates two logical NBMA networks (shown as circles) configured over a single physical NBMA network. Router A can communicate with routers B and C because they share the same network identifier (2). Router C can also communicate with routers D and E because they share network identifier 7. After address resolution is complete, router A can send IP packets to router C in one hop, and router C can send them to router E in one hop, as shown by the dotted lines.

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Figure 9 Two Logical NBMA Networks over One Physical NBMA Network

Destination

host

ip nhrp network-id 7

Router E

ip nhrp network-id 7 ip nhrp network-id 2

ip nhrp

network-id 2

Source

host

= Statically configured tunnel endpoints or permanent virtual circuits = Dynamically created virtual circuits

Router A

Router C

Router B ip nhrp network-id 2

Router D

ip nhrp network-id 7

S3230

The physical configuration of the five routers in Figure 9 might actually be that shown in Figure 10. The source host is connected to Router A and the destination host is connected to Router E. The same switch serves all five routers, making one physical NBMA network.

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IP Addressing Examples

Figure 10 Physical Configuration of a Sample NBMA Network

Source

host

Router A

Router E

Destination

host

Router B

Router C

Router D

S3231

Refer again to Figure 9. Initially, before NHRP has resolved any NBMA addresses, IP packets from the source host to the destination host travel through all five routers connected to the switch before reaching the destination. When Router A first forwards the IP packet toward the destination host, Router A also generates an NHRP request for the IP address of the destination host. The request is forwarded to Router C, whereupon a reply is generated. Router C replies because it is the egress router between the two logical NBMA networks.

Similarly, Router C generates an NHRP request of its own, to which Router E replies. In this example, subsequent IP traffic between the source and the destination still requires two hops to traverse the NBMA network, because the IP traffic must be forwarded between the two logical NBMA networks. Only one hop would be required if the NBMA network were not logically divided.

NHRP over ATM Example

The following example shows a configuration of three routers using NHRP over ATM. Subinterfaces and dynamic routing also are used. Router A obtains an OSPF route that it can use to reach the LIS where Router B resides. Router A can then initially reach Router B through Router C. Router A and Router B are able to directly communicate without Router C once NHRP has resolved the respective NSAP addresses of Router A and Router C.

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Configuring IP Addressing

The significant portions of the configurations for routers A, B, and C follow:

Router A Configuration

interface ATM0/0 ip address 10.1.0.1 255.255.0.0 ip nhrp network-id 1 map-group a atm nsap-address 11.1111.11.111111.1111.1111.1111.1111.1111.1111.11 atm rate-queue 1 10 atm pvc 1 0 5 qsaal

router ospf 1 network 10.0.0.0 0.255.255.255 area 0

map-list a

ip 10.1.0.3 atm-nsap 33.3333.33.333333.3333.3333.3333.3333.3333.3333.33

Router B Configuration

interface ATM0/0 ip address 10.2.0.2 255.255.0.0 ip nhrp network-id 1 map-group a atm nsap-address 22.2222.22.222222.2222.2222.2222.2222.2222.2222.22 atm rate-queue 1 10 atm pvc 2 0 5 qsaal

router ospf 1 network 10.0.0.0 0.255.255.255 area 0

map-list a

ip 10.2.0.3 atm-nsap 33.3333.33.333333.3333.3333.3333.3333.3333.3333.33

Router C Configuration

interface ATM0/0 no ip address atm rate-queue 1 10 atm pvc 2 0 5 qsaal

interface ATM0/0.1 multipoint ip address 10.1.0.3 255.255.0.0 ip nhrp network-id 1 map-group a atm nsap-address 33.3333.33.333333.3333.3333.3333.3333.3333.3333.33 atm rate-queue 1 10

interface ATM0/0.2 multipoint ip address 10.2.0.3 255.255.0.0 ip nhrp network-id 1 map-group b atm nsap-address 33.3333.33.333333.3333.3333.3333.3333.3333.3333.33 atm rate-queue 1 10

router ospf 1 network 10.0.0.0 0.255.255.255 area 0 neighbor 10.1.0.1 priority 1 neighbor 10.2.0.2 priority 1

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Cisco IOS IP Configuration Guide

Cisco 78-11741-02 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

About Cisco IOS Software Documentation

Documentation Objectives

Audience

Documentation Organization

Documentation Modules

Master Indexes

Supporting Documents and Resources

New and Changed Information

Document Conventions

Obtaining Documentation

World Wide Web

Documentation CD-ROM

Ordering Documentation

Documentation Feedback

Obtaining Technical Assistance

Cisco.com

Technical Assistance Center

Contacting TAC by Using the Cisco TAC Website

Contacting TAC by Telephone

Using Cisco IOS Software

Understanding Command Modes

Getting Help

Example: How to Find Command Options

Using the no and default Forms of Commands

Saving Configuration Changes

Filtering Output from the show and more Commands

Identifying Supported Platforms

Using Feature Navigator

Using Software Release Notes

IP Overview

IP Addressing and Services

IP Routing Protocols

Determining a Routing Process

Interior and Exterior Gateway Protocols

Interior Gateway Protocols

Exterior Gateway Protocols

Multiple Routing Protocols

IP Multicast

IP Addressing and Services

Configuring IP Addressing

IP Addressing Task List

Assigning IP Addresses to Network Interfaces

Assigning Multiple IP Addresses to Network Interfaces

Enabling Use of Subnet Zero

Disabling Classless Routing Behavior

Enabling IP Processing on a Serial Interface

Configuring Address Resolution Methods

Establishing Address Resolution

Defining a Static ARP Cache

Setting ARP Encapsulations

Enabling Proxy ARP

Configuring Local-Area Mobility

Mapping Host Names to IP Addresses

Assigning Host Names to IP Addresses

Specifying the Domain Name

Specifying a Name Server

Enabling the DNS

Using the DNS to Discover ISO CLNS Addresses

Configuring HP Probe Proxy Name Requests

Configuring the Next Hop Resolution Protocol

The Cisco Implementation of NHRP

Protocol Operation

NHRP Configuration Task List

Enabling NHRP on an Interface

Configuring a Static IP-to-NBMA Address Mapping for a Station

Statically Configuring a Next Hop Server

Configuring NHRP Authentication

Controlling the Triggering of NHRP

Triggering NHRP Based on Traffic Thresholds

Controlling the NHRP Packet Rate

Suppressing Forward and Reverse Record Options

Specifying the NHRP Responder Address

Changing the Time Period NBMA Addresses Are Advertised as Valid

Configuring a GRE Tunnel for Multipoint Operation

Configuring NHRP Server-Only Mode

Enabling IP Routing