Avaya IP Exterior Gateway Protocols User Manual
BayRS Version 15.0
Part No. 308628-15.0 Rev 00
June 2001
600 Technology Park Drive
Billerica, MA 01821-4130
Configuring IP Exterior Gateway Protocols (BGP and EGP)
Copyright © 2001 Nortel Networks
All rights reserved. June 2001.
The information in this document is subject to change without notice. The statements, configurations, technical data,
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308628-15.0 Rev 00
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Contents
Preface
Before You Begin ............................................................................................................. xv
Text Conventions .............................................................................................................xvi
Acronyms ........................... .......................... .......................... ......................... ...............xviii
Related Publications ........................................................................................................ xx
How to Get Help ..............................................................................................................xxi
Chapter 1
Exterior Gateway Protocols (BGP and EGP)
Autonomous Systems and Gateway Protocols ...............................................................1-1
Border Gateway Protocol (BGP) ..............................................................................1-3
Exterior Gateway Protocol (EGP) .............................................................................1-3
Classless Interdomain Routing ................................................................................1-4
BGP Concepts and Terminology .....................................................................................1-4
Peer-to-Peer Sessions .............................................................................................1-5
Stub and Multihomed Autonomous Systems ...........................................................1-6
Interior BGP Routing ... ...... ....... ...... ............................................. ....... ...... ....... ...... ...1-6
IBGP Route Reflector ...............................................................................................1-7
Equal-Cost Multipath ................................................................................................1-8
BGP Updates ...........................................................................................................1-8
Path Attributes ..........................................................................................................1-9
BGP/OSPF Interaction ...........................................................................................1-10
BGP-4 Confederations ...........................................................................................1-11
BGP-4 TCP MD5 Message Authentication ........................................................... .1-11
BGP Implementation Notes ..........................................................................................1-12
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Chapter 2
Starting BGP and EGP Services
Starting BGP with the BCC ............................................................................................2-1
Step 1: Configuring Global BGP ..............................................................................2-1
Step 2: Defining a Peer-to-Peer Connection ............................................................2-2
Starting IP and BGP with Site Manager .........................................................................2-3
Deleting BGP with Site Manager ..............................................................................2-4
Deleting BGP-3 and BGP-4 with Site Manager ........................................................2-5
Starting IP and EGP with Site Manager .........................................................................2-6
Deleting EGP from the Router .................................................................................2-7
Chapter 3
Configuring Global BGP Parameters
Disabling and Reenabling BGP ................................ ...... ....... .........................................3-2
Supplying a BGP Router ID ............................................................................................3-4
Identifying the Local AS ..................................................................................................3-5
Disabling and Reenabling IBGP Support .............................. ...... ...................................3-6
Specifying Route Types for IBGP Advertisements ..........................................................3-7
Enabling BGP Interaction with OSPF and RIP ...............................................................3-9
Setting the Update Interval Timer .................................................................................3-10
Allowing Redundant Connections ................................... ....... ...... ...... ...........................3-11
Enabling Multihop Connections ....................................................................................3-13
Disabling and Reenabling Dynamic Policy Configuration ............ ...... ...........................3-15
Configuring the BGP Soloist Slot Mask ........................................................................3-16
Disabling and Reenabling Route Aggregation ............................. ...... ....... ...... ....... ...... .3-17
Enabling and Disabling Black Hole Punching ...............................................................3-18
Disabling and Reenabling the BGP-4 MED Attribute ....................................................3-20
Configuring BGP-4 Confederations ..............................................................................3-21
Disabling BayRS Local Preference Calculation and Route Selection ...........................3-25
Calculating BGP-4 Local Preference Values ..........................................................3-25
Best-Route Selection ............................................... ....... ...... ...... ...........................3-27
Configuring BGP Message Logging .............................................................................3-29
Configuring EBGP Route Flap Damping ......................................................................3-31
Assigning Weight Classes and Values to an AS ...........................................................3-37
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Chapter 4
Configuring BGP Peers
Defining a Peer-to-Peer Session ....................................................................................4-2
Initiating a Peer-to-Peer Session ....................................................................................4-4
Negotiating the BGP Version ..........................................................................................4-6
Keeping the Connection Alive .........................................................................................4-8
Setting the External Advertisement Timer ......................................................................4-9
Specifying a Holddown Time ........................................................................................4-11
Setting a Minimum AS Origination Interval ...................................................................4-12
Overriding the Local AS Number ..................................................................................4-14
Specifying a Maximum Update Size .............................................................................4-14
Specifying a Time-to-Live Value ...................................................................................4-16
Specifying the Next-Hop Router ...................................................................................4-17
Setting the Route Echo Switch .....................................................................................4-18
Disabling and Reenabling Loop Detection ...................... ....... ...... .................................4-20
Configuring Peers over an Unnumbered Point-to-Point Link ........................................4-21
Configuring and Enabling MD5 Authentication .............................................................4-22
Entering and Storing MD5 Authentication Keys .....................................................4-23
Initializing TCP with the MD5 Option ......................................................................4-24
Generating MD5 Signatures on Transmitted BGP TCP Packets ............................4-24
Verifying MD5 Signatures on Received BGP TCP Packets ...................................4-25
Configuring BGP-4 Authentication .........................................................................4-25
Chapter 5
Configuring BGP Accept and Announce Policies
Configuring a BGP Accept Policy ...................................................................................5-2
Specifying Match Criteria for a BGP Accept Policy ..................................................5-6
Supplying Modification Values for a BGP Accept Policy ........................................5-10
Configuring a BGP Announce Policy ............................................................................5-13
Announce Policy Guidelines ...................................................................................5-13
Specifying Match Criteria for a BGP Announce Policy ...........................................5-16
Supplying Modification Values for a BGP Announce Policy ...................................5-22
Configuring BGP-4 AS Path Pattern-Matching .............................................................5-27
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Chapter 6
Configuring a Route Reflector
Configuring a Single Route Reflector in an AS ...............................................................6-3
Configuring a Route Reflector Cluster ............................................................................6-5
Configuring Multiple RR Clusters in an AS .....................................................................6-7
Configuring an RR Client ..............................................................................................6-10
Chapter 7
Configuring Route and Traffic Balancing
Configuring IBGP for Route and Traffic Balancing ..........................................................7-1
Configuring EBGP for Route and Traffic Balancing ........................................................7-5
Chapter 8
Customizing EGP Services
EGP Concepts and Terminology .....................................................................................8-2
EGP Implementation Notes ............................................................................................8-5
Customizing EGP on the Router ....................................................................................8-6
Enabling and Disabling EGP ....................................................................................8-6
Supplying a Local AS Number .................................................................................8-7
Configuring a Neighbor ...................................................................................................8-7
Specifying the Neighbor’s Address ..........................................................................8-8
Specifying the Gateway Mode ..................................................................................8-9
Enabling and Disabling the Neighbor Relationship ................................................8-10
Choosing the Acquisition Mode ..............................................................................8-11
Choosing the Poll Mode .........................................................................................8-12
Setting Neighbor Timers ........................................................................................8-13
Appendix A
Site Manager Parameters
BGP Parameters ............................................................................................................ A-2
BGP Configuration Parameters ............................................................................... A-3
BGP Global Parameters .......................................................................................... A-4
BGP-3 Global Parameter ...................................................................................... A-11
BGP-4 Global Parameter ...................................................................................... A-11
BGP Peer Parameters ........................................................................................... A-11
BGP AS Weight and Weight Class Parameters .................................................... A-20
BGP Event Message Parameters .. ....... ...... ....... ...... ....... ...... ...... ....... ................... A-24
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EGP Parameters .......................................................................................................... A-25
EGP Global Parameters ....................................................................................... A-25
EGP Neighbor Parameters .................................................................................... A-26
Routing Policy Par a meters .......................................................................................... A-29
Common Accept Policy Parameters ...................................................................... A-29
EGP-Specific Accept Policy Parameters ............................................................... A-34
BGP-3-Specific Accept Policy Parameters ........................................................... A-36
BGP-4-Specific Accept Policy Parameters ............................................................ A-40
Common Announce Policy Parameters ................................................................. A-50
EGP-Specific Announce Policy Parameters .......................................................... A-68
BGP-3-Specific Announce Policy Parameters ....................................................... A-70
BGP-4-Specific Announce Policy Parameters ....................................................... A-74
Appendix B
Converting Cisco to Nortel Networks Equivalents
Configuration Command Equivalents ............................................................................ B-1
Interpreting the Configuration Command Equivalents Table ......................................... B-6
Comparing the Operational Commands ................... ...... ....... ...... ...... ....... ...... ....... ...... .. B-8
Interpreting the Operational Commands Table .............................................................. B-9
Comparing BGP Route Selection Processes .............................................................. B-11
Regular Expression Symbols ...................... ...... ....... ...... ....... ...... ................................ B-12
Nortel Networks AS Path Pattern-Matching Symbols .................................................. B-13
Index
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Figures
Figure 1-1. Internetwork Segmented into Three Autonomous Systems .....................1-2
Figure 1-2. BGP Connecting Two Autonomous Systems Running OSPF ..................1-4
Figure 1-3. Transit AS .................................................................................................1-7
Figure 3-1. BGP Confederation ................................................................................3-23
Figure 4-1. Establishing and Confirming a Connection Between BGP Peers ............4-4
Figure 4-2. BGP over an Unnumbered Point-to-Point Link .......................................4-21
Figure 6-1. IBGP Single Route Reflector Topology ....................................................6-2
Figure 7-1. BGP/OSPF Autonomous System ............................................................7-2
Figure 7-2. IBGP ECMP Route Balancing ..................................................................7-3
Figure 7-3. IBGP ECMP Traffic Balancing ..................................................................7-4
Figure 7-4. ECMP Static Routes ................... ....... ...... ....... .........................................7-6
Figure 8-1. EGP Connection Between Two Autonomous Systems Running RIP .......8-2
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Tables
Table 1-1. BGP-3 Path Attributes ..............................................................................1-9
Table 1-2. BGP-4 Mandatory Path Attributes ............................................................1-9
Table 1-3. BGP-4 Optional Path Attributes .............................................................1-10
Table 3-1. Route Types for BGP Advertisements ......................................................3-8
Table 3-2. Slot Mask Parameter Values ..................................................................3-16
Table 3-3. Black Hole Punching Parameter Settings ..............................................3-19
Table 3-4. Best-Route Selection Rules ........................................ ....... ...... .............. 3 -27
Table 3-5. Local Preference Calculation Method ....................................................3-28
Table 3-6. Route Flap Damping Template Parameters ...........................................3-33
Table 4-1. MD5 Signature Verification Rules on BGP TCP Packets .......................4-25
Table 5-1. BCC Definition Parameters for BGP Accept Policies ...............................5-4
Table 5-2. BCC Match Parameters for BGP Accept Policies ....................................5-7
Table 5-3. BCC Modification Parameters for BGP Accept Policies .........................5-10
Table 5-4. BCC Definition Parameters for BGP Announce Policies ........................5-14
Table 5-5. BCC Match Parameters for BGP Announce Policies .............................5-17
Table 5-6. BCC Modification Parameters for BGP Announce Policies ....................5-22
Table 5-7. Characters in AS Path Pattern-Matching ...............................................5-27
Table 7-1. IBGP ECMP Methods ..............................................................................7-4
Table 7-2. EBGP ECMP Methods .............................................................................7-8
Table 8-1. Router Mode Determinator ......................................................................8-3
Table B-1. Cisco to Nortel Networks BGP Translation ............................................. B-2
Table B-2. Cisco and Nortel Networks BGP Operational Commands ...................... B-8
Table B-3. Route Selection Process Comparison .................................................. B-11
Table B-4. Regular Expression Symbols ................................................................ B-12
Table B-5. Nortel Networks AS Path Pattern-Matching Symbols ........................... B-13
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Preface
Routers at the borders or edg es of a utonomous s ystems a re call ed gateways. These
gateways use exterior gateway protocols to exchange rea chability information
with each other and route packets between routing domains. This guide describes
how to configure and use the IP Border Gateway Prot ocol (BGP) and the Exte rior
Gateway Protocol (EGP).
You can use the Bay Command Console (BCC*) or Site Manager to configure
BGP and EGP on a router. In this guide, you will find instructions for using both
the BCC and Site Manager.
Before You Be gin
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).
• Connect the router to the network and create a pilot configuration file (see
Quick-Starting Routers, Configuring Remote Access for AN and Passport
ARN Routers
Make sure that you are running the latest version of Nortel Networks* BayRS*
and Site Manager software. For information about upgrading BayRS and Site
Manager, see the upgrading guide for your version of BayRS.
308628-15.0 Rev 00
, or
Connecting ASN Routers to a Network)
.
xv
Configuring IP Exterior Gateway Protocols (BGP and EGP)
Text Conventions
This guide uses the following text conventions:
angle brackets (< >) Indicate that you choose the text to enter based on the
description inside the brackets. Do not type the
brackets when entering the command.
Example: If the command syntax is:
ping
<ip_address>
ping 192.32.10.12
, you enter:
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.
}.
braces ({}) Indicate required elements in syntax descriptions
where there is more than one option. You must choose
only one of the options. Do not type the braces when
entering the command.
Example: If the command syntax is:
show ip {alerts | routes
show ip alerts or show ip routes
}
, you must enter either:
, but not both.
brackets ([ ]) Indicate optional elements in syntax descriptions. Do
not type the brackets when entering the command.
Example: If the command syntax is:
show ip interfaces [-alerts
show ip interfaces
or
]
, you can enter either:
show ip interfaces -alerts
.
ellipsis points (. . . ) Indicate that you repeat the last element of the
command as needed.
xvi
Example: If the command syntax is:
ethernet/2/1
ethernet/2/1
[<parameter> <value>]
and as many parameter-value pairs as
needed.
. . .
, you enter
308628-15.0 Rev 00
Preface
italic text Indicates new terms, book titles, and variables in
command syntax descri pti ons. Where a v a ri abl e is two
or more 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 > IP identifi es the IP option on t h e
Protocols menu.
vertical line (
) Separates choices for command keywords and
|
arguments. Enter only one of the choices. Do not type
the vertical line when entering 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.
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Configuring IP Exterior Gateway Protocols (BGP and EGP)
Acronyms
This guide uses the following acronyms:
ARP Address Resolution Protocol
AS autonomous system
ATM asynchronous transfer mode
BGP Border Gatew a y Protoc ol
CIDR classless interdomain rout ing
DES data encryption standard
EBGP Exterior Border Gateway Protocol
ECMP equal-cost multipath
EGP Exterior Gateway Protocol
FDDI Fiber Distributed Data Interface
HSSI High Speed Serial Interface
IBGP Interior Border Gateway Protocol
xviii
ICMP Internet Control Message Protocol
IGP interior gateway protocol
IP Internet Protocol
ISP Internet service provider
LAN local area network
MD5 M essage Digest 5
MED multiexit discriminator
MEK message encryption key
MIB management information base
MSS maximum segment size
NLRI network layer reachability information
NPK node protection key
NVRAM nonvolatile random access memory
OSPF Open Shortest Path First
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Preface
PPP Point-to-Point Protocol
PVC permanent virtual circuit
RARP Reverse Address Resolution Protocol
RFC request for comments
RIP Routing Information Protocol
RR route reflector
SMDS Switched Multimegabit Data Service
TCP Transmission Control Protocol
UDP User Datagram Protocol
WAN wide area network
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Configuring IP Exterior Gateway Protocols (BGP and EGP)
Related Publications
For more information about IP services, refer to the following publications:
• Reference for BCC IP show Commands (part number 308603-14.20 Rev 00)
show
Provides descriptions of all
commands that display BGP configuration and statistical data.
• Configuring IP, ARP, RARP, RIP, and OSPF Services (part number
308627-15.0 Rev 00)
Provides a description of IP, ARP, RARP, RIP, and OSPF services and
instructions for configuring them.
• Configuring GRE, NAT, RIPSO, and BFE Services (part number
308625-14.20 Rev 00)
Provides a description of Generic Routing Encapsulation (GRE), Network
Address Translation (NAT), Revised IP Security Option (RIPSO), and Blacker
front-end services and instructions for configuring them.
You can print selected technical manuals and release notes free, directly from the
Internet. Go to the www.nortelnetworks.com/documentation URL. Find the
product for which you ne ed do cumen tat ion. Then locate the speci fic category and
model or version for your hardware or software product. Use Adobe* Acrobat
Reader* to open the manuals and release notes, search for the sections you need,
and print them on most standard printers. Go to Adobe Systems at the
www.adobe.com URL to download a free copy of the Adobe Acrobat Reader.
commands for IP services, including the
xx
You can purchase selected documentation sets, CDs, and technical publications
through the Internet at the www1.fatbrain.com/documentation/nortel/ URL.
308628-15.0 Rev 00
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 w orks s ervic e progr am, cont act on e of t he fol lo win g
Nortel Networks Technical Solutions Centers:
Technical Solutions Center Telephone
Europe, Middle East, and Africa (33) (4) 92-966-968
North America (800) 4NORTEL or (800) 466-7835
Asia Pacific (61) (2) 9927-8800
China (800) 810-5000
An Express Routing Code (ERC) is available for many Nortel Networks products
and services. When you use an ERC, your call is routed to a technical support
person who speciali zes in supp orting t hat product or servi ce. To locate an ERC for
your product or service, go to the www12.nortelnetworks.com/ URL and click
ERC at the bottom of the page.
Preface
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xxi
Chapter 1
Exterior Gateway Protocols (BGP and EGP)
This chapter introduces the concepts and terminology used in this guide.
Topic Page
Autonomous Systems and Gateway Protocols 1-1
BGP Concepts and Terminology 1-4
BGP Implementation Notes 1-12
Autonomous Systems a nd Gateway Protocols
LANs and WANs interconnected by IP routers for m a group of netw orks call ed an
internetwork. For administrative purposes, an internetwork is divided into
autonomous systems. An autonomous system (AS) is a group of routers (called
gateways in IP terminolog y) and host s run by a singl e techni cal admi nistr ator th at
has a single, clearly defined routing policy. Each autonomous system has its own
unique AS number as signe d by the app ropri ate In ter net Regi stry entit y. Figure 1-1
shows a sample internetwork segmented into three autonomous systems.
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1-1
Configuring IP Exterior Gateway Protocols (BGP and EGP)
LAN
A
Router
1
Autonomous
system 1
LAN
F
Autonomous
system 2
LAN
C
Router
9
Router
2
Router
8
LAN
G
LAN
B
Router
3
Router
4
Router
Autonomous
system 3
Router
7
LAN
E
5
LAN
D
Router
6
Figure 1-1. Internetwork Segmented into Three Autonomous Systems
The routers at the edges (or borders) of autonomous systems are called gateways.
These gate ways use exterior gatewa y protocols to exchange reachability
information and to route packets between routing domains.
1-2
IP0006B
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Border Gateway Protocol (BGP)
The Border Gateway Protocol (BGP) is an exterior gateway protocol used by
border routers to exchange network reachability information with other BGP
systems. BGP routers form peer relationships with other BGP routers in other
autonomous systems or within the same autonomous system. BGP peers transmit
and receive current ro uting in form ation over a reli able tr anspor t la yer conne ction,
making periodic updates unnecessary. BGP is designed for inter-AS exchanges,
but can be used between multiple routing domains (for example, RIP to OSPF).
BGP peers exchange complete routing information only after the peer connection
is established. Thereafter, BGP peers exchange routing updates. An update
includes a network number, a list of autonomous systems that the routing
information passed through (the AS path), and other path attributes that describe
the route to a set of dest inati on net works. When mul tipl e path s are avail able, BGP
compares the path attributes to choose the preferred path.
BGP exchanges information between ASs as well as between routers in the same
AS. To differentiate between these uses, the latter is called interior BGP (IBGP).
Exterior Gateway Protocols (BGP and EGP)
Exterior Gateway Protocol (EGP)
You use the Exterior Gateway Protocol to exchange network reachability
information between routers in different autonomous systems. An interior
gateway protocol (IGP), such as RIP or OSPF, is used with in an AS to facilitate
the communication of routing information within an autonomous system. The
routers that serve as the end points of a connection between two autonomous
systems also run an exterior gateway protocol, such as EGP-2.
Routers establish EGP neighbor relationships to periodically exchange reliable
network reachability inf ormation. EGP neig hbors exchange complete reachability
information, not just upda tes. T he rout er us es this infor mat ion to mainta in a li st of
gateways, the networks the gateways can reach, and the corresponding distances.
Chapter 8, “
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Customizing EGP Services,” describes the use of EGP.
1-3
Configuring IP Exterior Gateway Protocols (BGP and EGP)
Classless Interdomain Routing
Classless interdomain routing (CIDR) is an addressing scheme that uses supernet
addresses to represent multiple IP destinations. Rather than advertise a separate
route for each destination network in a supernet, a router uses a supernet address
to advertise a single route (called an aggregate route) that represents all the
destinations. CIDR reduces the size of the routing tables used to store advertised
IP routes. BGP-4 supports classless interdomain routing.
BGP Concepts and Terminology
BGP is an exterior gateway protocol designed to exchange network reachability
information with other BGP systems in other autonomous systems or within the
same autonomous system.
Figure 1-2
shows two autonomous systems: AS1 and AS2. Networks within AS1
and AS2 are connected by routers running an interior gateway protocol—in this
case, OSPF. The two ASs are connected by routers that run an exterior gateway
protocol—BGP—in addition to OSPF.
OSPF
AS1
OSPF
OSPF
OSPF/
BGP
Exterior
BGP
connection
OSPF/
BGP
AS2
OSPF
OSPF
IP00025A
Figure 1-2. BGP Connecting Two Autonomous Systems Running OSPF
1-4
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Exterior Gateway Protocols (BGP and EGP)
Nortel Networks supports BGP-3 and BGP-4:
• BGP-3 assumes that eac h adver tised networ k is a n atura l clas s netwo rk (A , B,
or C), based on its high-order bits. BGP-3 cannot advertise subnets or
supernets.
• BGP-4 has no concept of address classes. Each network listed in the network
layer reachability information (NLRI) portion of an update message contains
a prefix length field, which describes the length of the mask associated with
the network. The prefix length field allows for both supernet and subnet
advertisement. The supernet advertisement is what makes classless
interdomain routing (CIDR) possible. See “Class le ss Int erdomain Routing”
on page 1-4.
In addition, BGP-4 supports BGP confederations and TCP MD5 message
authentication.
This sect i on covers the following topics:
Topic Page
Peer-to-Peer Sessions 1-5
Stub and Multihomed Autonomous Systems 1-6
Interior BGP Routing 1-6
IBGP Route Reflector 1-7
Equal-Cost Multipath 1-8
BGP Updates 1-8
Path Attributes 1-9
BGP/OSPF Interaction 1-10
BGP-4 Confederations 1-11
BGP-4 TCP MD5 Message Authentication 1-11
Peer-to-Peer Sessions
A BGP router employs a BGP speaker, which is an entity within the router that
transmits and receives BGP messages and acts upon them. A BGP speaker forms
a neighbor relationship with another BGP speaker by establishing a peer-to-peer
session. See Chapter 4, “
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Configuring BGP Peers.”
1-5
Configuring IP Exterior Gateway Protocols (BGP and EGP)
Stub and Multihomed Autonomous Systems
An AS can include one or more BGP speakers t hat e st abl is h peer-to-peer ses si ons
with BGP speakers in other autonomous systems to provide external route
information for the networks within the A S. A multihomed AS has multiple BGP
speakers. A stub AS has a single BGP speaker that establishes a peer-to-peer
session with one external BGP speaker. The BGP speaker provides external route
information only for the networks contained within its own AS.
Interior BGP Routing
Nortel Networks implements interior BGP (IBGP) intra-AS routing. With IBGP,
each router in the AS runs an interior gateway protocol (IGP), such as OSPF, for
internal routing updates and also maintains an IBGP connection to each BGP
border router. The IBGP information, along with the IGP route to the originating
BGP border router, determines the next hop to use for external networks.
Some IGPs carry no BGP information. However, an OSPF type 5 LSA can carry
BGP-specific information in its tag field. Each router uses IBGP exclusively to
determine reachability to external networks. When an IBGP update for a network
is received, it is passed to IP for inclusio n in the routing table only if a viable IGP
route to the correct border gateway is available.
1-6
An AS with more than one BGP spe aker c an use I BGP to p rovide a tra nsit s ervic e
for networks outside the AS. An AS that provides this service is called a transit
AS (Figure 1-3
).
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Exterior Gateway Protocols (BGP and EGP)
AS 10
AS 20
AS 50
Figure 1-3. Transit AS
BGP A
IGP
BGP C
AS 30
AS 11
AS 12
BGP B
IP0021A
In Figure 1-3, AS 20 is the transit AS. It provides information about its internal
networks, as well as transit networks, to the remaining ASs. The IBGP
connections between BGP routers A, B, and C provide consistent routing
information to the ASs.
IBGP Route Reflector
A BGP router configured for IBGP establishes a peer-to-peer session with every
other IBGP speaker in the AS. In an AS with a large number of IBGP speakers,
this full-mesh topology can result in high bandwidth and maintenance costs. For
example, a full-mesh topology for an AS with 50 IBGP speakers requires 1225
internal peer-to-peer connections.
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Configuring IP Exterior Gateway Protocols (BGP and EGP)
T o avoid the high cost s of a full-me sh topology to support IBGP speakers wit hin a
large AS, you can configure a router to function as an IBGP route reflector (RR).
An IBGP speaker that needs to communicate with other BGP speakers in the AS
establishes a single peer-to-peer RR client session with the IBGP route reflector.
For information about the IBGP route reflector, see Chapter 6, “Configuring a
Route Reflector.”
Equal-Cost Multipath
Equal-cost multipath (ECMP) support allows a BGP speaker to perform route or
traffic balancing within an AS by using multiple equal-cost routes submitted to
the routing table by OSPF, RIP, or static routes. For instructions on configuring
route and traffic balancing, see Chapter 7, “
Balancing.” For more information ab out equal- cost multi path, see Configuring IP,
ARP, RARP, RIP, and OSPF Services.
BGP Updates
BGP-3 and BGP-4 speakers exchange routing updates that include a network
number and a list of autonomous systems that the routing information has passed
through (the AS path) as well as a list of unreachable networks. In addition, an
update includes the following:
Configuring Route and Traffic
1-8
• List of path attributes
• Local preference value—BGP-4 only. (See “
Preference Calculation and Route Selection” on page 3-25.)
Disabling BayRS Local
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