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Chapter 29 Configuring IP Unicast Routing
Configuring BGP
The network has these characteristics:
•
Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP
peers are directly connected and that the IBGP peers are not. As long as there is an IGP running that
allows the two neighbors to reach one another, IBGP peers do not have to be directly connected.
•
All BGP speakers within an AS must establish a peer relationship with each other. That is, the BGP
speakers within an AS must be fully meshed logically. BGP4 provides two techniques that reduce
the requirement for a logical full mesh: confederations and route reflectors.
•
AS 200 is a transit AS for AS 100 and AS 300—that is, AS 200 is used to transfer packets between
AS 100 and AS 300.
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 29-48
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.
•
Default BGP Configuration, page 29-43
•
Enabling BGP Routing, page 29-45
•
Managing Routing Policy Changes, page 29-47
•
Configuring BGP Decision Attributes, page 29-48
•
Configuring BGP Filtering with Route Maps, page 29-50
•
Configuring BGP Filtering by Neighbor, page 29-51
•
Configuring Prefix Lists for BGP Filtering, page 29-52
•
Configuring BGP Community Filtering, page 29-53
•
Configuring BGP Neighbors and Peer Groups, page 29-55
•
Configuring Aggregate Addresses, page 29-57
•
Configuring Routing Domain Confederations, page 29-57
•
Configuring BGP Route Reflectors, page 29-58
•
Configuring Route Dampening, page 29-59
•
Monitoring and Maintaining BGP, page 29-60
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.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)EY.”
