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Chapter 36 Configuring IP Unicast Routing
Configuring BGP
BGP peers initially exchange their full BGP routing tables and then send only incremental updates. BGP
peers also exchange keepalive messages (to ensure that the connection is up) and notification messages
(in response to errors or special conditions).
In BGP, each route consists of a network number, a list of autonomous systems that information has
passed through (the autonomous system path), and a list of other path attributes. The primary function
of a BGP system is to exchange network reachability information, including information about the list
of AS paths, with other BGP systems. This information can be used to determine AS connectivity, to
prune routing loops, and to enforce AS-level policy decisions.
A router or switch running Cisco IOS does not select or use an IBGP route unless it has a route available
to the next-hop router and it has received synchronization from an IGP (unless IGP synchronization is
disabled). When multiple routes are available, BGP bases its path selection on attribute values. See the
“Configuring BGP Decision Attributes” section on page 36-49 for information about BGP attributes.
BGP Version 4 supports classless interdomain routing (CIDR) so you can reduce the size of your routing
tables by creating aggregate routes, resulting in supernets. CIDR eliminates the concept of network
classes within BGP and supports the advertising of IP prefixes.
These sections contain this configuration information:
Default BGP Configuration, page 36-43
Enabling BGP Routing, page 36-45
Managing Routing Policy Changes, page 36-47
Configuring BGP Decision Attributes, page 36-49
Configuring BGP Filtering with Route Maps, page 36-51
Configuring BGP Filtering by Neighbor, page 36-51
Configuring Prefix Lists for BGP Filtering, page 36-53
Configuring BGP Community Filtering, page 36-54
Configuring BGP Neighbors and Peer Groups, page 36-55
Configuring Aggregate Addresses, page 36-57
Configuring Routing Domain Confederations, page 36-58
Configuring BGP Route Reflectors, page 36-58
Configuring Route Dampening, page 36-59
Monitoring and Maintaining BGP, page 36-60
For detailed descriptions of BGP configuration, see the “Configuring BGP” chapter in the “IP Routing
Protocols” part of the Cisco IOS IP Configuration Guide, Release 12.2. For details about specific
commands, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.
Locate these documents from the Cisco.com page under Documentation > Cisco IOS Software > 12.2
Mainline > Configuration Guides or Command References.
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(50)SE.”