Cisco Systems 32369 User Manual

Cisco IOS DHCP Server

Feature Overview

Dynamic Host Control Protocol (DHCP) enables you to automatically assign reusable IP addresses to DHCP clients. The Cisco IOS DHCP Server feature is a full DHCP server implementation that assigns and manages IP addresses from speciﬁed address pools within the router to DHCP clients. If the Cisco IOS DHCP Servercannot satisfy a DHCP request from its own database, it can forward the request to one or more secondary DHCP servers deﬁned by the network administrator.

Figure 1 shows the basic steps that occur when a DHCP client requests an IP address from a DHCP server. The client, Host A, sends a DHCPDISCOVER broadcast message to locate a Cisco IOS DHCP Server. A DHCP server offers conﬁguration parameters (such as an IP address, a MAC address, a domain name, and a lease for the IP address) to the client in a DHSCPOFFER unicast message.

Figure 1 DHCP Request for an IP Address from a DHCP Server

Host A

Note A DHCP client may receive offers from multiple DHCP servers and can accept any one of the

offers; however, the client usually accepts the ﬁrst offer it receives. Additionally, the offer from the DHCP server is not a guarantee that the IP address will be allocated to the client; however, the server usually reserves the address until the client has had a chance to formally request the address.

The client returns a formal request for the offered IP address to the DHCP server in a DHCPREQUEST broadcast message. The DHCP server conﬁrms that the IP address has been allocated to the client by returning a DHCPACK unicast message to the client.

DHCPDISCOVER (broadcast)

DHCPOFFER (unicast)

DHCPREQUEST (broadcast)

DHCPACK (unicast)

Cisco IOS

DHCP server

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Cisco IOS DHCP Server 1

Benefits

Beneﬁts

Note The formal request for the offered IP address (the DHCPREQUEST message) that is sent by

the client is broadcast so that all other DHCP servers that received the DHCPDISCOVERbroadcast message from the client can reclaim the IP addresses that they offered to the client.

If the conﬁguration parameters sent to the client in the DHCPOFFER unicast message by the DHCP server are invalid (a misconﬁguration error exists), the client returns a DHCPDECLINE broadcast message to the DHCP server.

The DHCP server will send to the client a DHCPNAK denial broadcast message, which means the offered conﬁguration parameters have not been assigned, if an error has occurred during the negotiationof the parameters or the client hasbeen slow in responding to the DHCPOFFER message (the DHCP server assigned the parameters to another client) of the DHCP server.

The Cisco IOS DHCP Server feature offers the following beneﬁts:

• Reduced Internet access costs

Using automatic IP address assignment at each remote site substantially reduces Internet access costs. Static IP addresses are considerably more expensive to purchase than are automatically allocated IP addresses.

• Reduced client conﬁguration tasks and costs

Because DHCP is easy to conﬁgure, it minimizes operational overhead and costs associated with device conﬁguration tasks and eases deployment by nontechnical users.

• Centralized management

Because the DHCP server maintains conﬁgurations for several subnets, an administrator only needs to update a single, central server when conﬁguration parameters change.

Supported Platforms

This Cisco IOS DHCP Server feature is supported on the following routers and access servers:

• Catalyst 5000 family switches with an installed Route Switch Module

• Catalyst 6000 family switches with an installed MultiLayer Switch Feature Card

• Catalyst 8500 series

• Cisco 800 series

• Cisco 1000 series

• Cisco 1400 series

• Cisco 1600 series

• Cisco 1700 series (support for the Cisco 1700 series was added in Cisco IOS Release 12.0[2]T)

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• Cisco 2500 series

• Cisco 2600 series

• Cisco 3600 series

• Cisco 3800 series

• Cisco MC3810 series

• Cisco 4000 series

• Cisco AS5100 access server

• Cisco AS5200 universal access server

• Cisco AS5300 universal access server

• Cisco 7000 series

• Cisco 7100 series

• Cisco 7200 series

• Cisco MGX 8800 with an installed Route Processor Module

• Cisco 12000 series

• Cisco uBR900 series

• Cisco uBR7200 series

Supported Standards, MIBs, and RFCs

Standards

No new or modiﬁed standards are supported by this feature.

MIBs

No new or modiﬁed MIBs are supported by this feature. For descriptions of supported MIBs and how to use MIBs, see the Cisco MIB web site on CCO at

http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.

RFCs

• RFC 951, Bootstrap Protocol (BOOTP)

• RFC 1542, Clariﬁcations and Extensions for the Bootstrap Protocol

• RFC 2131, Dynamic Host Conﬁguration Protocol

• RFC 2132, DHCP Options and BOOTP Vendor Extensions

Cisco IOS DHCP Server 3

Prerequisites

Before you conﬁgure the Cisco IOS DHCP Server feature, complete the following tasks:

• Identify an external File Transport Protocol (FTP), Trivial File Transfer Protocol (TFTP), or

remote copy protocol (rcp) server that you will use to store the DHCP bindings database.

• Identify the IP addresses that you will enable the DHCP server to assign, and the IP addresses

that you will exclude.

• Identify DHCP options for devices where necessary, including:

— Default boot image name — Default router(s) — Domain Name System (DNS) server(s) — NetBIOS name server

• Decide on a NetBIOS node type (b, p, m, or h).

• Decide on a DNS domain name.

DHCP Conﬁguration Task List

The DHCP server database is organized as a tree. The root of the tree is the address pool for natural networks, branches are subnetwork address pools, and leaves are manual bindings to clients. Subnetworks inherit network parameters and clients inherit subnetwork parameters. Therefore, common parameters, for example the domain name, should be conﬁgured at the highest (network or subnetwork) level of the tree.

Note Inherited parameters can be overridden. For example, if a parameter is deﬁned in both the

natural network and a subnetwork, the deﬁnition of the subnetwork is used.

Address leases are not inherited. If a lease is not speciﬁed for an IP address, by default, the DHCP server assigns a one-day lease for the address.

To conﬁgure the Cisco IOS DHCP Server feature, ﬁrst conﬁgure a database agent or disable conﬂict logging, then conﬁgure IP addresses that the DHCP server should not assign (excluded addresses) and should assign (a pool of available IP addresses) to requesting clients. These conﬁguration tasks are explained in the following sections. Each task in the following list is identiﬁed as required or optional.

• Conﬁguring a DHCP Database Agent or Disabling DHCP Conﬂict Logging (Required)

• Excluding IP Addresses (Required)

• Conﬁguring a DHCP Address Pool (Required)

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• Conﬁguring Manual Bindings (Optional)

• Conﬁguring a DHCP Server Boot File (Optional)

• Conﬁguring the Number of Ping Packets (Optional)

• Conﬁguring the Timeout Value for Ping Packets (Optional)

• Enabling the Cisco IOS DHCP Server Feature (Optional)

Configuring a DHCP Database Agent or Disabling DHCP Conflict Logging

Conﬁguring a DHCP Database Agent or Disabling DHCP Conﬂict Logging

A DHCP database agent is any host, for example, an FTP, TFTP, or RCP server that stores the DHCP bindings database. You can conﬁgure multiple DHCP database agents and you can conﬁgure the interval between database updates and transfers for each agent. To conﬁgure a database agent and database agent parameters, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp database url [timeout seconds | write-delay seconds]

If you choose not to conﬁgure a DHCP database agent, disable the recording of DHCP address conﬂicts on the DHCP server. To disable DHCP address conﬂict logging, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# no ip dhcp conﬂict logging

Conﬁgures the database agent and the interval between database updates and database transfers.

Disables DHCP address conﬂict logging.

Excluding IP Addresses

The DHCP server assumes that all IP addresses in a DHCP address pool subnet are available for assigning to DHCP clients. You must specify the IP address that the DHCP server should not assign to clients. To do so, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp excluded-address low-address

[high-address]

Speciﬁes the IP addresses that the DHCP server should not assign to DHCP clients.

Conﬁguring a DHCP Address Pool

You can conﬁgure a DHCP address pool with a name that is a symbolic string (such as “engineering”) or an integer (such as 0). Conﬁguring a DHCP address pool also places you in DHCP pool conﬁguration mode—identiﬁed by the (conﬁg-dhcp)# prompt—from which you can conﬁgure pool parameters (for example, the IP subnet number and default router list). To conﬁgure a DHCP address pool, complete the required tasks in the following sections.

Conﬁguring the DHCP Address Pool Name and Entering DHCP Pool Conﬁguration Mode

To conﬁgure the DHCP address pool name and enter DHCP pool conﬁguration mode, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp pool name Creates a name for the DHCP server address pool and places

you in DHCP pool conﬁguration mode (identiﬁed by the conﬁg-dhcp# prompt).

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DHCP Configuration Task List

Conﬁguring the DHCP Address Pool Subnet and Mask

To conﬁgure a subnet and mask for the newly created DHCP address pool, which contains the range of available IP addresses that the DHCP server may assign to clients, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# network network-number [mask | /preﬁx-length]

Note You can not conﬁgure manual bindings within the same pool that is conﬁgured with the

Speciﬁes the subnet network number and mask of the DHCP address pool.

The preﬁx length speciﬁes the number of bits that comprise the address preﬁx. The preﬁx is an alternative way of specifying the network mask of the client. The preﬁx length must be preceded by a forward slash (/).

network command. To conﬁgure manual bindings, see the “Conﬁguring Manual Bindings” section.

Conﬁguring the Domain Name for the Client

The domain name of a DHCP client places the client in the general grouping of networks that make up the domain. To conﬁgure a domain name string for the client, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# domain-name

domain

Speciﬁes the domain name for the client.

Conﬁguring the Domain Name System IP Servers for the Client

DHCP clients query DNS IP servers when they need to correlate host names to IP addresses. To conﬁgure the DNS IP servers that are available to a DHCP client, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# dns-server address [address2 ... address8]

Speciﬁes the IP address of a DNS server that is available to a DHCP client. One IP address is required; however, you can specify up to eight IP addresses in one command line.

Conﬁguring the NetBIOS Windows Internet Naming Service IP Servers for the Client

WindowsInternet Naming Service(WINS) is a name resolutionservice that Microsoft DHCP clients use to correlate host names to IP addresses within a general grouping of networks. Toconﬁgure the NetBIOS WINS servers that are available to a Microsoft DHCP client, use the following command in DHCP pool conﬁguration mode:

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Command Purpose

Router(conﬁg-dhcp)#netbios-name-server address [address2 ... address8]

Speciﬁes the NetBIOS WINS server that is available to a Microsoft DHCP client. One address is required; however, you can specify up to eight addresses in one command line.

Configuring the NetBIOS Node Type for the Client

Conﬁguring the NetBIOS Node Type for the Client

The NetBIOS node type for Microsoft DHCP clients can be one of four settings: broadcast, peer-to-peer,mixed, or hybrid. Toconﬁgure the NetBIOS node type for a Microsoft DHCP,use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# netbios-node-type

type

Speciﬁes the NetBIOS node type for a Microsoft DHCP client.

Conﬁguring the Default Router for the Client

Aftera DHCP client has booted, the client begins sending packetsto its default router.The IP address of the default router should be on the same subnet as the client. To conﬁgure a default router for a DHCP client, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# default-router address [address2 ... address8]

Speciﬁes the IP address of the default router for a DHCP client. One IP address is required, although you can specify up to eight addresses in one command line.

Conﬁguring the Address Lease Time

By default, each IP address assigned by a DHCP server comes with a one-day lease, which is the amount of time that the address is valid. To change the lease value for an IP address, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# lease {days [hours][minutes] | inﬁnite}

Conﬁguring Manual Bindings

An address binding is a mapping between the IP address and Media Access Control (MAC) address of a client. The IP address of a client can be assigned manually by an administrator or assigned automatically from a pool by a DHCP server.

Manual bindings are IP addresses that have been manually mapped to the MAC addresses of hosts that are found in the DHCP database. Manual bindings are stored in NVRAM on the DHCP server. Manual bindings are just special address pools. There is no limit on the number of manual bindings but you can only conﬁgure one manual binding per host pool.

Automatic bindings are IP addresses that have been automatically mapped to the MACaddresses of hosts that are found in the DHCP database. Automatic bindings are stored on a remote host called a database agent. The bindings are saved as text records for easy maintenance.

To conﬁgure a manual binding, ﬁrst create a host pool, then specify the IP address and hardware address of the client or client identiﬁer. The hardware address is the MAC address. The client identiﬁer, which is required for Microsoft clients (instead of hardware addresses), is formed by concatenating the media type and the MAC address of the client. Refer to the “Address Resolution Protocol Parameters” section of RFC 1700, Assigned Numbers, for a list of media type codes.

Speciﬁes the duration of the lease. The default is a a one-day lease.

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DHCP Configuration Task List

Toconﬁgure manual bindings, use the following commands beginningin global conﬁguration mode:

Step Command Purpose

1 Router(conﬁg)# ip dhcp pool name Creates a name for the a DHCP server address

2 Router(conﬁg-dhcp)# host address [mask |

3 Router(conﬁg-dhcp)# hardware-address

4 Router(conﬁg-dhcp)# client-name name (Optional) Speciﬁes the name of the client using

/preﬁx-length]

hardware-address type

or Router(conﬁg-dhcp)# client-identiﬁer

unique-identiﬁer

pool and places you in DHCP pool conﬁguration mode—identiﬁed by the (conﬁg-dhcp)# prompt.

Speciﬁes the IP address and subnet mask of the client.

The preﬁx length speciﬁes the number of bits that comprise the address preﬁx. The preﬁx is an alternativeway of specifying the network mask of the client. The preﬁx length must be preceded by a forward slash (/).

Speciﬁes a hardware address for the client.

Speciﬁes the distinct identiﬁcation of the client in dotted-hexadecimal notation, for example, 01b7.0813.8811.66, where 01 represents the Ethernet media type.

any standard ASCII character. The client name should not include the domain name. For example, the name mars should not be speciﬁed as mars.cisco.com.

Configuring a DHCP Server Boot File

The boot ﬁle is used to store the boot image for the client. The boot image is generally the operating system the client uses to load. Tospecifya boot ﬁle for the DHCP client, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# bootﬁle ﬁlename Speciﬁes the name of the ﬁle that is used as a boot image.

Conﬁguring the Number of Ping Packets

By default, the DHCP server pings a pool address twice before assigning the address to a requesting client. If the ping is unanswered, the DHCP server assumes (with a high probability) that the address is not in use and assigns the address to the requesting client. To change the number of ping packets the DHCP server should send to the pool address before assigning the address, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp ping packets

number

Speciﬁes the number of ping packets the DHCP server sends to a pool address before assigning the address to a requesting client. The default is two packets.

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Configuring the Timeout Value for Ping Packets

Conﬁguring the Timeout Value for Ping Packets

By default, the DHCP server waits 500 milliseconds before timing out a ping packet. To change the amount of time the server waits, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp ping timeout

milliseconds

Speciﬁes the amount of time the DHCP server must wait before timing out a ping packet. The default 500 milliseconds.

Enabling the Cisco IOS DHCP Server Feature

By default, the Cisco IOS DHCP Server feature is enabled on your router. If the feature is disabled, use the following command in global conﬁguration mode to reenable the Cisco IOS DHCP Server feature on your router:

Command Purpose

Router(conﬁg)# service dhcp Enables the Cisco IOS DHCP Server feature on your router.

Use the no form of this command to disable the Cisco IOS DHCP Server feature.

Monitoring and Maintaining the DHCP Server

To clear DHCP server variables, use the following commands in privileged EXEC mode, as needed:

Command Purpose

Router# clear ip dhcp binding address | * Deletes an automatic address binding from the DHCP

database. Specifying address clears the automatic binding for a speciﬁc (client) IP address whereas specifying asterisk (*) clears all automatic bindings.

Router# clear ip dhcp conﬂict address | * Clears an address conﬂict from the DHCP database. Specify-

ing address clears the conﬂict for a speciﬁc IP address whereas specifying an asterisk (*) clears conﬂicts for all addresses.

Router# clear ip dhcp server statistics Resets all DHCP server counters to 0.

To enable DHCP server debugging, use the following command in privileged EXEC mode, as needed:

Command Purpose

Router# debug ip dhcp server {events | packets | linkage}

Enables debugging on the DHCP server.

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Configuration Examples

To display DHCP server information, use the following commands in EXEC mode, as needed:

Command Purpose

Router> show ip dhcp binding [address] Displays a list of all bindings created on a speciﬁc DHCP

Router> show ip dhcp conﬂict [address] Displays a list of all address conﬂicts recorded by a speciﬁc

Router# show ip dhcp database [url] Displays recent activity on the DHCP database.

Router> show ip dhcp server statistics Displays count information about server statistics and mes-

Conﬁguration Examples

This section provides the following conﬁguration examples:

• DHCP Database Agent Conﬁguration Example

• DHCP Address Pool Conﬁguration Example

server.

DHCP server.

Note Use this command in privileged EXEC mode.

sages sent and received.

• Manual Bindings Conﬁguration Example

DHCP Database Agent Conﬁguration Example

The following example stores bindings on host 172.16.4.253. The ﬁle transfer protocol is FTP.The server should wait 2 minutes (120 seconds) before writing database changes.

ip dhcp database ftp://user:password@172.16.4.253/router-dhcp write-delay 120

DHCP Address Pool Conﬁguration Example

In the following example, three DHCP address pools are created: one in network 172.16.0.0, one in subnetwork172.16.1.0,and one in subnetwork 172.16.2.0. Attributesfromnetwork172.16.0.0, such as the domain name, DNS server, NetBIOS name server, and NetBIOS node type, are inherited in subnetworks 172.16.1.0 and 172.16.2.0. In each pool, clients are granted 30-day leases and all addresses in each subnetwork, except the excluded addresses, are available to the DHCP server for assigning to clients. Table 1 lists the IP addresses for the devices in three DHCP address pools.

Table 1 DHCP Address Pool Devices Pool 0 (Network 172.16.0.0) Pool 1 (Subnetwork 172.16.1.0) Pool 2 (Subnetwork 172.16.2.0)

Device IP Address Device IP Address Device IP Address

Default routers – Default routers 172.16.1.100

172.16.1.101

DNS server 172.16.1.102 — — — —

172.16.2.102

NetBIOS name server 172.16.1.103 — — — —

172.16.2.103

NetBIOS node type h-node — — — —

Default routers 172.16.2.100

172.16.2.101

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ip dhcp database ftp://user:password@172.16.4.253/router-dhcp write-delay 120 ip dhcp excluded-address 172.16.1.100 172.16.1.103 ip dhcp excluded-address 172.16.2.100 172.16.2.103 ! ip dhcp pool 0

network 172.16.0.0 /16 domain-name cisco.com dns-server 172.16.1.102 172.16.2.102 netbios-name-server 172.16.1.103 172.16.2.103

netbios-node-type h-node ! ip dhcp pool 1

network 172.16.1.0 /24

default-router 172.16.1.100 172.16.1.101

lease 30 ! ip dhcp pool 2

network 172.16.2.0 /24

default-router 172.16.2.100 172.16.2.101

lease 30

Manual Bindings Conﬁguration Example

The following example creates a manual binding for a client named Mars.cisco.com. The MAC address of the client is 02c7.f800.0422 and the IP address of the client is 172.16.2.254.

ip dhcp pool Mars

host 172.16.2.254

hardware-address 02c7.f800.0422 ieee802

client-name Mars

Manual Bindings Configuration Example

Because attributes are inherited, the previous conﬁguration is equivalent to the following:

ip dhcp pool Mars

host 172.16.2.254 mask 255.255.255.0

hardware-address 02c7.f800.0422 ieee802

client-name Mars

default-router 172.16.2.100 172.16.2.101

domain-name cisco.com

dns-server 172.16.1.102 172.16.2.102

netbios-name-server 172.16.1.103 172.16.2.103

netbios-node-type h-node

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Command Reference

This section documents new or modiﬁed commands. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command reference publications.

• bootﬁle

• clear ip dhcp binding

• clear ip dhcp conﬂict

• clear ip dhcp server statistics

• client-identiﬁer

• client-name

• default-router

• dns-server

• domain-name

• hardware-address

• host

• ip dhcp conﬂict logging

• ip dhcp database

• ip dhcp excluded-address

• ip dhcp ping packets

• ip dhcp ping timeout

• ip dhcp pool

• ip dhcp relay information check

• ip dhcp relay information option

• ip dhcp relay information policy

• lease

• netbios-name-server

• netbios-node-type

• network (DHCP)

• next-server

• option

• service dhcp

• show ip dhcp binding

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• show ip dhcp conﬂict

• show ip dhcp database

• show ip dhcp server statistics

Manual Bindings Configuration Example

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

command | {begin | include | exclude} regular-expression Following is an example of the show atm vc command in which you want the command output to

begin with the ﬁrst line where the expression “PeakRate” appears: show atm vc | begin PeakRate For more information on the search and ﬁlter functionality, refer to the Cisco IOS Conﬁguration

Fundamentals Conﬁguration Guide.

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Command Reference

bootﬁle

Syntax Description

Defaults

Command Modes

Tospecify the name of the defaultboot image for a Dynamic Host Configuration Protocol (DHCP) client, use the bootﬁle DHCP pool conﬁguration command. Use the no form of this command to delete the boot image name.

bootﬁle ﬁlename no bootﬁle

ﬁlename Speciﬁes the name of the ﬁle that is used as a boot image.

No default behavior or values.

DHCP pool conﬁguration

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

The following example speciﬁes xllboot as the name of the boot ﬁle:

bootfile xllboot

ip dhcp pool next-server

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clear ip dhcp binding

To delete an automatic address binding from the Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server database, use the clear ip dhcp binding privileged EXEC command.

clear ip dhcp binding address | *

Syntax Description

address The address of the binding you want to clear.

* Clears all automatic bindings.

Command Modes

Privileged EXEC

Usage Guidelines

Typically, the address denotes the client’s IP address. If the asterisk (*) character is used as the address parameter, DHCP clears all automatic bindings.

clear ip dhcp binding

Examples

Related Commands

Use the no ip dhcp pool global conﬁguration command to delete a manual binding.

The following example deletes the address binding 10.12.1.99 from a DHCP server database:

clear ip dhcp binding 10.12.1.99

show ip dhcp binding

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Command Reference

clear ip dhcp conﬂict

To clear an address conﬂict from the Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server database, use the clear ip dhcp conﬂict privileged EXEC command.

clear ip dhcp conﬂict address | *

Syntax Description

address The IP address of the host that contains the conﬂicting address you

* Clears all address conﬂicts.

Command Modes

Privileged EXEC

want to clear.

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. The server detects conﬂicts using a ping session. The client detects conﬂicts using gratuitous

Address Resolution Protocol (ARP). If the asterisk (*) character is used as the address parameter, DHCP clears all conﬂicts.

The following example shows an address conﬂict of 10.12.1.99 being deleted from the DHCP server database:

clear ip dhcp conflict 10.12.1.99

show ip dhcp conﬂict

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