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Chapter 38
Configuring IP Unicast Routing
Configuring BGP
Configuring BGP
The Border Gateway Protocol (BGP) is an exterior gateway protocol for an interdomain routing system
that guarantees the loop-free exchange of routing information between autonomous systems.
Autonomous systems are made up of routers operating under the same administration and run Interior
Gateway Protocols (IGPs), such as RIP or OSPF, within their boundaries and interconnecting by using
an Exterior Gateway Protocol (EGP). BGP Version 4 is the standard EGP for interdomain routing in the
Internet. You can find detailed information about BGP in Internet Routing Architectures, published by
Cisco Press, and in the "Configuring BGP" chapter in the Cisco IP and IP Routing Configuration Guide.
For details about BGP commands and keywords, see the "IP Routing Protocols" part of the Cisco IOS
IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2. For a list of BGP commands
that are visible but not supported by the switch, see
Appendix C, "Unsupported Commands in
Cisco IOS Release 12.2(52)SE."
Routers belonging to the same autonomous system and exchanging BGP updates run internal BGP
(IBGP). Routers belonging to different autonomous systems and exchanging BGP updates run external
BGP (EBGP). Most configuration commands are the same for configuring EBGP and IBGP. The
difference is that the routing updates are exchanged either between autonomous systems (EBGP) or
within an autonomous system (IBGP).
Figure 38-5
shows a network that is running both EBGP and
IBGP.
Figure 38-5
EBGP , IBGP , and Multiple Autonomous Systems
AS 100
Router A
Router D
AS 300
129.213.1.2
192.208.10.1
EBGP
EBGP
129.213.1.1
192.208.10.2
IBGP
Router B
Router C
175.220.212.1
175.220.1.2
AS 200
Before exchanging information with an external autonomous system, BGP ensures that networks in the
autonomous system can be reached by defining internal BGP peering among routers and by
redistributing BGP routing information to IGPs that run in the autonomous system, such as IGRP and
OSPF.
Routers that run a BGP routing process are often referred to as BGP speakers. BGP uses the
Transmission Control Protocol (TCP) as its transport protocol (specifically port 179). Two BGP speakers
that have a TCP connection to each other are known as peers or neighbors. In
Figure
38-5, Routers A
and B are BGP peers, as are Routers B and C and Routers C and D. The routing information is a series
of autonomous-system numbers that describe the full path to the destination network. BGP uses this
information to construct a loop-free map of autonomous systems.
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
38-46
OL-12247-04
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