Avaya ATM Half-Bridge User Manual
BayRS Version 14.00
Part No. 308611-14.00 Rev 00
September 1999
4401 Great America Parkway
Santa Clara, CA 95054
Configuring ATM Half-Bridge Services
Copyright © 1999 Nortel Networks
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308611-14.00 Rev 00
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308611-14.00 Rev 00
Contents
Preface
Before You Begin .............................................................................................................xiii
Text Conventions .............................................................................................................xiv
Acronyms ........................... .......................... .......................... ......................... .................xvi
Hard-Copy Technical Manuals . ....... ...... ....... ............................................. ...... ....... ........xviii
How to Get Help ............................................................................................................xviii
Chapter 1
ATM Half-Bridge Overview
What Is ATM Half Bridge? ..............................................................................................1-1
How the AHB Router Works ...........................................................................................1-4
Forwarding Inbound Packets from CPE Hosts .........................................................1-4
Forwarding Outbound Packets to CPE Hosts ..........................................................1-5
Using DHCP to Dynamically Assign IP Addresses ..................................................1-5
Dynamically Learning New Bridge Table Entries .....................................................1-7
Unsecure Learning Method ...............................................................................1-8
Secure Learning Method ...................................................................................1-8
Maintaining the AHB Bridge Table ...........................................................................1-9
Adding Bridge Table Entries ..............................................................................1-9
Removing Bridge Table Entries .......................................................................1-10
Replacing Bridge Table Entr ies ....... ...... ....... ...... ....... .......................................1-10
Using Source-Based Routing .................................................................................1-10
Responding to Proxy ARP Requests .....................................................................1-11
Using Inbound Packet Filtering ..............................................................................1-12
308611-14.00 Rev 00
v
Chapter 2
Starting AHB Services
Network Planning Considerations ..................................................................................2-1
AHB Configuration Requirements ..................................................................................2-1
Preparing a Configuration File .................................................................................2-2
Configuring an IP Interface on the Router ................................................................2-3
Configuring Circuitless IP on the Router ..................................................................2-4
Configuring AHB Services ..............................................................................................2-5
Creating an ATM Interface ........................................................................................2-6
Adding an LLC/SNAP PVC Service Record ............................................................2-6
Adding AHB to the LLC/SNAP Service Record .......................................................2-7
Adding PVCs to the LLC/SNAP PVC Service Record .............................................2-9
Configuring a DHCP/BootP Relay Agent ......................................................................2-10
Creating a BootP Relay Agent Forwarding Interfaces ............................................2-10
Configuring BootP Preferred Servers .....................................................................2-12
Specifying the Relay Agent IP Address ...........................................................2-12
Specifying the Target Server IP Address .........................................................2-13
Chapter 3
Customizing AHB Services
Modifying the AHB Configuration ...................................................................................3-1
Enabling and Disabling AHB ....................................................................................3-1
Setting the Auto Learn Method ................................................................................3-2
Configuring a Local AHB Init File .............................................................................3-3
Sample Init File ..................................................................................................3-3
Specifying an Alternate Init File ...............................................................................3-5
Specifying the Debug Level ......................................................................................3-6
Enabling and Disabling Inbound Packet Filtering .....................................................3-7
Modifying AHB Circuits ...................................................................................................3-8
Enabling and Disabling an AHB Circuit ....................................................................3-8
Specifying the Subnet Mask for the Learned Host ...................................................3-9
Enabling and Disabling Proxy ARP on a Circuit .....................................................3-10
Configuring Traffic Filters for Source-Based Routing ....................................................3-10
Deleting AHB from the Router ......................................................................................3-11
vi
308611-14.00 Rev 00
Appendix A
AHB, BootP/DHCP, IP, and ATM
Parameter Descriptions
AHB Global Parameters ................................................................................................ A-2
AHB Circuit Parameters ................................................................................................. A-6
BootP Relay Agent Interface Table Parameters ............................................................. A-8
BootP Address Parameters ......................................................................................... A-10
BootP Relay Agent Preferred Server Table Parameters .............................................. A-12
BootP Preferred Server Configuration Parameters ...................................................... A-14
IP Configuration Parameters .................................... ............................................. ...... A-15
ATM Interface Parameter Descriptions ..................... ...... ....... ...... ................................ A-17
ATM Service Record Parameter Descriptions ............................................................. A-19
ATM Virtual Channel Link Parameter Descriptions ...................................................... A-23
Appendix B
Understanding ATM Concepts
ATM General Information ............................................................................................... B-1
ATM Cells ................................................................................................................ B-1
Cell Headers ..................................................................................................... B-1
ATM Data Transmission ........................................................................................... B-3
Permanent Virtual Connections .............................................................................. B-5
Service Records and Virtual Circuits ............................................................................. B-5
Data Encapsulation ....................................................................................................... B-5
Assigning a Data Encapsulation Type for a PVC .................................................... B-6
PVC Access Methods .................................................................................................... B-6
Multiple PVCs .......................................................................................................... B-7
One PVC ................................................................................................................. B-8
Index
308611-14.00 Rev 00
vii
Figures
Figure 1-1. DSL Network with an AHB Router ...........................................................1-3
Figure 1-2. How CPE Hosts Identify a DCHP Server .................................................1-6
Figure 1-3. How CPE Hosts Request and Receive IP Addresses from
the DHCP Server .....................................................................................1-7
Figure A-1. Edit ATM Half-Bridge Global Parameters Window .................................. A-2
Figure A-2. ATM Half-Bridge Circuit List .................................................................... A-6
Figure A-3. BOOTP Relay Agent Interface Table Window ......................................... A-8
Figure A-4. BOOTP Addresses Window .................................................................. A-10
Figure A-5. BOOTP Relay Agent Preferred Server Table Window .......................... A-12
Figure A-6. BOOTP Preferred Server Configuration Window .................................. A-14
Figure A-7. Interface Attributes Window Sequence ................................................. A-17
Figure A-8. Service Attributes Window Sequence (PVCs) ...................................... A-20
Figure B-1. ATM Cell .................................................................................................. B-2
Figure B-2. ATM Transmission Components .............................................................. B-4
Figure B-3. Multiple PVCs per Service Record ......................................................... B-7
Figure B-4. One PVC per Service Record ................................................................. B-9
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ix
Tables
Table 1-1. Sample AHB Bridge Table .......................................................................1-9
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xi
This guide describes asynchronous transfer mode (ATM) Half-Bridge services
and what you do to start and customize ATM Half-Bridge se rvices on a Nortel
Networks
™
Before You Begi n
Before using this guide, you must complete the following procedures. For a new
router:
• Install the router (see the installation guide that came with your router).
Preface
router.
• Connect the router to the network and create a pilot configuration file (see
Make sure that you are runni ng the lates t versio n of Nortel Netw orks BayRS
Site Manager software. For information about upgrading BayRS and Site
Manager, see the upgrading guide for your version of BayRS.
308611-14.00 Rev 00
Quick-Starti ng Router s , Conf igur ing BaySt ac k Remote Acc ess , or Connecting
ASN Routers to a Network).
™
and
xiii
Configuring ATM Half-Bridge Services
Text Con ventions
This guide uses the following text conventions:
bold text
Indicates command names and options and text that
you need to enter.
Example: Enter
Example: Use the
show ip {alerts | routes
dinfo
command.
}.
italic text Indicates file and directory names, new terms, book
titles, and variables in command syntax descriptions.
Where a variable is two or mor e words, the words are
connected by an underscore.
Example: If the command syntax is:
show at
valid_route
<
valid_route
>
is one variable and you substitute one value
for it.
screen text Indicates system output, for example, prompts and
system messages.
Example:
Set Trap Monitor Filters
separator ( > ) Shows menu paths.
Example: Protocols > I P ide nti fies the IP option on the
Protocols menu.
xiv
vertical line (
) Separates choices for command keywords and
|
arguments. Enter only one of the choices. Do not type
the vertical line when enteri ng the command.
Example: If the command syntax is:
show ip {alerts | routes
show ip alerts
or
}
, you enter either:
show ip routes
, but not both.
308611-14.00 Rev 00
Acronyms
Preface
This manual uses the follow i ng acr onyms:
AAL ATM adaptation layer
ADSL asymmetric digital subscriber line
AHB ATM Half Bridge
ARE ATM Routing Engine
ARP Address Resolution Protocol
ATM Asynchronous Transfer Mode
ATU-C ADSL terminal unit, central
ATU-R ADSL terminal unit, remote
AUI Attachment Unit Interface
BootP Bootstrap Protocol
BRI Basic Rate Interface
CO central office
CCITT International Telegraph and Telephone Consultative
DHCP Dynamic Host Configuration Protocol
DSL Digital Subscriber Line
CPE customer premise equipment
DSLAM DSL access multiplexer
DTE data terminal equipment
GUI graphical user interface
HDLC high-level data li nk control
ILI Intelligent Link Interface
IP Internet P rotocol
ISDN Integrated S ervices Digital Network
ISO International Organizat ion for Stand ardization
ISP internet service provider
308611-14.00 Rev 00
Committee (now ITU-T)
xv
Configuring ATM Half-Bridge Services
ITU-T International Telecommunications
LAN local area network
MAC media access control
MAU media access unit
MIB management information base
MTU Maximum Transmis sion Unit
OSI Open Systems Interconnection
PDU protocol data unit
PVC permanent virtual circuit
RIP Routing Information Protocol
SAP service access point
SDU service data unit
SNAP Subnetwork Access Protocol
STAT status
Union-Telecommunications (formerly CCITT)
xvi
SVC switched virtual circuit
TCP/IP Transmission Control Protocol/Internet Protocol
UTP unshielded twisted-pair
VC virtual circuit
VCI virtual connection id entifier
VCL virtual channel link
VPI virtual path identifier
WAN wide area network
308611-14.00 Rev 00
Hard-Copy Technical Manuals
You can print selected technical manuals and release notes free, directly from the
Internet. Go to support.baynetworks.com/library/tpubs/. Find the product for
which you need documentation. Then locate the specific category and model or
version for your hardw are or soft ware product . Usi ng Adobe Ac robat Re ader, you
can open the manuals and releas e notes, search for the sections you ne ed, and print
them on most standard printers. You can download Acrobat Reader free from the
Adobe Systems Web site, www.adobe.com.
You can purchase selected documentation sets, CDs, and technical publications
through the collateral catalog. The catalog is located on the World Wide Web at
support.baynetworks.com/catalog.html and is divided into sections arranged
alphabetically:
• The “CD ROMs” section lists available CDs.
• The “Guides/Books” section lists books on technical topics.
• The “Technical Manuals” section lists available printed documentation sets.
Preface
How to Get Help
If you purchased a service contract for your Nortel Networks product from a
distributor or authorized reseller, contact the technical support staff for that
distributor or reseller for assistance.
If you purchased a Nort el Net wor ks s ervice pr ogram, c ontact one of the f ollowing
Nortel Networks Technical Solutions Centers:
Technical Solutions Center Telephone Number
Billerica, MA 800-2LANWAN (800-252-6926)
Santa Clara, CA 800-2LANWAN (800-252-6926)
Valbonne, France 33-4-92-96-69-68
Sydney, Australia 61-2-9927-8800
Tokyo, Japan 81-3-5402-7041
308611-14.00 Rev 00
xvii
Chapter 1
ATM Half-Bridge Overview
This chapter provides an overview of basic ATM Half-Bridge concepts and the
specific ways Nortel Networks implements these concepts on BLN
routers.
This chapter assumes that you understand ATM concepts and terminology. For a
brief overview of ATM, see Appendix B. For detailed information about ATM,
refer to Configuring ATM Services.
®
and BCN®
This chapter contains information about the following topics:
Topic Page
What Is ATM Half Bridge? 1-1
How the AHB Router Works 1-4
What Is ATM Half Bridge?
ATM Half Bridge (AHB) is a protocol operating on BLN and BCN routers that
connect remote hosts ( PCs and w orks tati ons) at tached to an IP route d net work vi a
digital subscrib er line (DSL) devices (modems). The modems send and receiv e I P
packets over ATM PVCs using RFC 1483 SNAP/LLC encapsulation. AHB
converts RFC 1483 bridge frames containing IP packets to unencapsulated routed
frames and performs the reverse function for packets returning from the opposite
direction.
308611-14.00 Rev 00
1-1
Configuring ATM Half-Bridge Services
Nortel Networks designed AHB to meet the requirements of public network
providers who want to develop an end-to-end solution using DSL to provide
high-speed internet an d remot e LAN acce ss t o ISP’s and corporate networks. The
AHB router supports the public network provider’s WAN infrastructure network
by performing the bi dire ction al conversion of RFC 1483 Ethernet bridge d pack ets
for IP routing to and from the ISP and corporate networks.
Figure 1-1
shows a sample DSL access network that uses an AHB router. This
network is divided into four separate segments:
• The customer premises, which consist of th e remote host and modem, referred
to as ADSL terminal unit, remote end devices (ATU-R)
• The network provider POP, which consists of DSL access multiplexor ADSL
terminal unit, central devices (DSLAM ATU-C), ATM access network, and
the AHB router
• The public network WAN infrastructure (typically IP over ATM or frame
relay)
• The ISP or corporate network, which provides internet service for remote
CPE stations
1-2
308611-14.00 Rev 00
0050
ATM Half-Bridge Overview
Remote
customer premises
IP
802.3
PC1
PC2
IP
802.3
AAL
ATM
ATU-R
ATM
over
DSL
Network provider
central office
DSLAM
ATU-C
M
U
X
DSLAM
1483 encapsulated IP
packets in ATM
IP
802.3
AAL
ATM
OC-3 link
BLN/BCN
router
Frames
Figure 1-1. DSL Network with an AHB (BLN/BCN) Router
WAN
infrastructure
IP
IP
ATM
Frame relay
ISP or
corporate network
ISP1
Corp1
Corp2
ISP2
ATM
A
The following steps explain how IP packets travel from th e CPE host (remote
user) on an Ethernet segment to the AHB router located at the public network
provider’s central office:
1.
2.
3.
308611-14.00 Rev 00
A user (PC or workstation) at a remote site sends Ethernet packets to an
ATU-R attached to a remote Ethernet LAN segment (10 MB).
The ATU-R, acting as an Ethernet/ATM bridge, performs the following tasks:
• Accepts the Ethernet packets
• Segments the Ethernet packets into ATM cells according to RFC 1483
(802.2/LLC bridge encapsulation)
• Transmits the ATM cells to a DSLAM ATU-C at the public network
provider’s site
The ATU-C receives the ATM cells, which are multiplexed along with cells
from other ATU-Cs, and tra nsmits them t o the ATM netw ork o v er one or more
OC-3 links.
1-3
Configuring ATM Half-Bridge Services
4.
The ATM network, located at the central office, switches the ATM cells and
forwards them over PVCs to the ATM interface on the AHB (BLN/BCN)
router.
How the AHB Router W orks
AHB combines bridging and I P routin g on the r outer pla tform. On t he DSL access
side, the router receives RFC 1483 encapsulated bridged Ethernet frames
containing IP packets from CPE hosts. On the WAN side, ISP’s and corporate
networks receive routed IP frames. AHB dynamically learns the association
between IP addresses assigned to CPE hosts and the correct RFC 1483 bridged
Ethernet header data for the host.
Forwarding Inbound Packets from CPE Hosts
AHB decapsulates and then routes IP packets it receives from a remote CPE host
to an ISP or corporate network. When AHB receives the incoming packets on its
ATM inte rface, it checks its local b ridge table to determine whether the p acket’s
source IP address corresponds to the host IP address in the associated AHB table
entry. If the bridge table contains the bridge entry, and the entry matches the
bridge information (the correct circuit and VCID), then AHB strips off the RFC
1483 bridged Ethernet header information and forwards the packet using IP
routing to the appropriate ISP’s and corporate networks.
1-4
If AHB does not find the bridge entry in its local bridge table, and you selected
unsecure learning mode, AHB creates a new bridge entry using the packet header
data from the incoming packet. If you selected secure learning mode, AHB
creates a new bridge entry only if this is a new DHCP request (for information
about using unsecure and secure learning mode, see “Dynamically Learning New
Bridge Table Entries”). The AHB router cannot use this bridge entry until it
receiv es a DHCP reply on a non-AHB interf ace that matches the desti nation MAC
address in the bridge ent ry. The IP address is assig ned at this time. If no matching
bridge entry is found for an i ncoming pack et on an AHB interf ace, the pack et may
still be forwarded if either it is a DHCP packet or if you have disabled inbound
packet filtering.
308611-14.00 Rev 00
Forwarding Outbound Packets to CPE Hosts
IP packets received from ISPs or corporate networks that are destined for CPE
hosts are sent to AHB b ased on th e AHB rout e in t he IP r outin g tabl e. When AHB
receiv es I P pack et s, it sea rches its loc al bridge t able fo r an asso cia ted bri dge ent ry
based on the packet’s destination IP address. If AHB finds an associated bridge
entry, it adds the RFC 1483 encapsulation and Ethernet header information to the
packet. Then it forwards the packet on the correct outgoing interface and PVC to
reach the remote CPE host.
If AHB does not f ind a n associ ated bri dge entr y for the pack et in its br idge ta ble, it
drops the packet.
Using DHCP to Dynamically Assign IP Addresses
CPE hosts can use DHCP to obtain their IP address and default gateway address.
When you boot the router, the CPE host sends a DHCP request packet, which is
relayed by the DSL modem to the AHB router. The AHB router acts as a DHCP/
BootP relay agent. When the BootP relay agent configured on the ATM service
record receives the DHCP request packet, it fills in the associated IP interface
address (the address you specified for the ATM interface) as the gateway address.
The BootP relay agent then forwards the packet to the DHCP server using either
the BootP forwarding interface method or the BootP preferred server method.
ATM Half-Bridge Overview
When the DHCP server receives the DHCP REQUEST packet, it checks to see
whether the gateway address is filled in. If this address is fille d in, DHCP sends a
reply packet (DHCPACK) to the BootP relay agent on the AHB router. The AHB
router then forw ar ds t he DHCPACK reply to the CPE ho st usi ng th e a sso ciated IP
address of the unnumbered IP interface.
DHCP servers often use the gateway address in the DHCP request to determine
the address pool to use when assigning a client IP address.
The DHCP server may not respond if it receives a DHCP request where the
gateway address is on the same subnet as the DHCP server’s IP address.
Therefore, when selecting an associated IP address for the unnumbered ATM
circuit, choose an add re ss other than the IP a ddr ess of a di re ctl y conn ect ed DHCP
server.
308611-14.00 Rev 00
1-5
Configuring ATM Half-Bridge Services
Figure 1-2 shows how the remote hosts identify the DHCP server on the network.
Remote
host
1. Remote host broadcasts
a DHCPDISCOVER
PC1
PC2
PC2
ATM
packet.
4. BootP relay agent
forwards
DHCPOFFER
message to
CPE host.
AHB router
(BootP relay agent)
DHCP
server
Figure 1-2. How CPE Hosts Identify a DCHP Server
2. BootP relay agent transmits
DHCPDISCOVER
packet to DHCP server
that supplied a
DHCPOFFER packet.
3. DHCP server
transmits
DHCPOFFER
message to BootP
relay agent in
AHB router.
ISP1
Corp1
Corp2
ISP2
ATM0053A
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Figure 1-3 shows how the CPE hosts request and receive IP addresses from the
DHCP server.
308611-14.00 Rev 00
ATM Half-Bridge Overview
Remote
host
PC1
PC2
PC2
ATM
1. Remote host
broadcasts
DHCPREQUEST
packet.
4. BootP relay agent
agent transmits
DHCPACK
packet or
DHCPNAK
packet to
remote host.
AHB router
(BootP relay agent)
DHCP
server
2. BootP relay
agent forwards
DHCPREQUEST
packet to DHCP
server using the
preferred server
or forwarding
interface method.
3. Target DHCP
server transmits
DHCPACK
packet or
DHCPNAK
packet to BootP
relay agent.
ISP1
Corp1
Corp2
ISP2
ATM0054A
Figure 1-3. How CPE Hosts Request and Receive IP Addresses from the DHCP Server
Dynamically Learning New Bridge Table Entries
The AHB router dynamically learns new bridge entries it receives from CPE
hosts. You can determine the method by which AHB learns these entries by
configuring the auto learning method configuration parameters on the AHB
router. AHB provides the following auto learning methods:
• Unsecure
•Secure
• Both (default)
•None
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Configuring ATM Half-Bridge Services
Unsecure Learning Method
AHB learns new bridge entries from any IP and ARP packets it receives on
ATM/AHB-configured inter faces when you set the auto learn method to unsec ure
or both.
Secure Learning Method
When you set the auto learning met hod to Secure or Both, AHB learns ne w bridge
entries from DHCP replies sent on AHB-configured circuits. Secure entries take
precedence over unsecure entries.
Before you can use secure learning mode, make sure that you:
• Configure BootP on both the unnumbered ATM interface and on the
associated numbered IP interface.
• Configure a BootP preferred server or forwarding interface on the
unnumbered ATM interface (use the circuit number as the IP source address).
• V e rify that the IP ad dress assigne d by the DHCP serv er is on a dif ferent subnet
than the BootP gateway address (that is, the associated numbered IP
interface).
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In secure learning mode, the AHB router serves as a BootP relay agent,
forwarding DHCP requests it receives from CPE hosts.
When you set the learning method to both, AHB learns bridge entries either
securely and unsecurely. When you set the learning method to None, AHB will
not learn new bridge entries.
For information about configuring the auto learn method on an AHB router, see
“Setting the Auto Learn Method”on page 3-2.
308611-14.00 Rev 00
Maintaining the AHB Bridge Table
AHB maintains a bridge t able o n each s lot. The tabl e cont ains a lis t of h ost en tries
to or from which the router will forward traffic. These host entries map the IP
address of a CPE host to the following:
• ATM interface on which it is learned on the AHB router
• Virtual Path Identi fier /Virtual Channel Ident ifie r (VPI/VCI) number
• RFC 1483 encapsulation type
• Ethe rnet header information
ATM Half-Bridge Overview
Table 1-1
Table 1-1. Sample AHB Bridge Table
Host ID
200.10.10.1 2 101/31 Bridged Ethernet frame:
shows a sample AHB bridge table.
Outgoing
Circuit VPI/VCI RFC 1483 Bridge Header
src MAC addr= 010203040506
dest MAC adrr= 07080oaaboc
Ethertype= 0800 (IP)
Adding Bridge Table Entries
In unsecure learning mode, AHB adds a new entry to the bridge table whenever it
receives an IP or ARP packet that is not already in the table.
In secure learning mode, AHB adds new entries to its table when it receives
certain incoming and outgoing DHCP messages. AHB adds new table entries
when the CPE host on the remote site s ends a DHCPDI SCOVER message (source
IP address equals 0.0.0.0). This new table entry identifies the incoming port,
VPI/VCI, and RFC 1483 encapsulation data (including source MAC address).
There is no IP address fo r the new table entry at this point.
AHB forwards only certain DHCP packets (ACK or NAK) to the CPE host using
this entry before an IP address is assigned. When AHB receives a DHCPACK
message, it updates the table entry to id entify the cor rect IP addr ess as provi ded by
the DHCP server. When AHB learns the first host entry on a network, it inserts a
route into the IP routing table for that network.
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1-9
Configuring ATM Half-Bridge Services
Removing Bridge Table Entries
AHB removes an existing table entry whenever it adds a new entry with the same
source MA C a ddress , ATM port, and VPI/VCI n umber. AHB also removes a ta ble
entry when it receives a DHCPRELEASE message (assuming that the source IP
address-to-ATM port /VPI /VCI ma ppi ng is correct). If the entry b ein g re moved is
on the original sl ot , al l other slots that h ave received copies ar e notified t o r emove
those entries. When AHB removes the last host entry on a network, it deletes the
corresponding route from the IP routing table.
Replacing Bridge Table Entries
If the bridge table contains a bridge entry that was learned on a different circuit/
PVC, and AHB learns another IP packet with the same IP address on a different
circuit and VCID, then AHB replaces the old entry with the new entry. New
entries that AHB learns in unsecure mode do not replace existing entries learned
in secure mode, even if AHB learned them on different slots.
Using Source-Based Routing
The network provider can use source-based routing to ensure that the packet sent
from a CPE ho st travels to the correct ISP gateway first, regardless o f its
destination address. The ISP gateway can be an edge router directly connected to
an AHB router via a WAN interface (frame relay, PPP) or a router located within
the ISP’s domain. This option allows multiple links connecting the AHB router
with the ISP network. If one link or edge router fails, traffic from ATM-attached
hosts is routed via another edge router.
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You use source-based routing by configuring an IP traffic filter on the ATM
interface. You must specify traffi c filter criteria and a traffic filter action. The
traffic filter criteria is the range of source IP add resses allocated to CPE hosts
belonging to a single ISP. You must configure a separate traffic filter for each
range of source addresses that get assigned to the ATM-attached hosts by an ISP.
These filters must be installed on every AHB circuit.
The traffic filter action is “forward to next hop,” wh ich identifies the ISP’s
gateway device. The next-hop address in the traffi c filter need not be a directly
attached host. If it is not a directly attached host, AHB accesses the routing table
to determine the directly attached host to which the packets are sent.
308611-14.00 Rev 00
You can use the “forward to next hop” feature to improve reliability. Consider an
example where an ISP home network has two border routers: 200.1.2.3 and
200.3.4.5. If the source-based routing gateway address (next-hop address in the
traffic filter) is advertised by both border routers, t hen the AHB ro uter will send
packets from the ATM-attached hosts to whichever border router has the best
route. If this border router fails or if the link goes down, then the packets are sent
to the alternate border router. The gateway address does not need to belong to a
real device because the ISP border router forwards packets based on the
destination IP address.
For information about configuring IP filters for source-based routing, see
“Configuring Traffic Filters for Source-Based Routing,” on page 3-10.
Responding to Proxy ARP Requests
By default, AHB receives ARP requests from ATM-attached hosts. When the
AHB router receives an ARP request, it responds to this request with its own
hardware address as the target MAC address. You must enable proxy ARP
separately on each circuit.
ATM Half-Bridge Overview
When proxy ARP is enabled, the AHB router responds to ARP requests if all of
the following conditions are true:
• The ARP request receiv ed by AHB is not a self-ARP request (tha t is , the ho st
• The host’s (sender) address is already in the bridge table or can be learned
• The target host address either is in the host table and has a different
For informat ion about conf iguring p roxy ARP, see “Enabling and Disabl ing Proxy
ARP on a Circuit” on page 3-10.
308611-14.00 Rev 00
IP address equals the target IP address or the source address equals 0.0.0.0).
from this packet (unsecure mode enabled).
circuit VPI/VCI, or the target host address is not in the bridge table.
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