
Configuring and Customizing BGP
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8-5
In Figure 8-2, 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 are necessary to provide consistent
information to the ASs.
IBGP Route Reflector
A BGP router configured for internal BGP (IBGP) must establish 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 1,225 internal peer-to-peer connections.
To avoid the high costs of a full-mesh topology to support IBGP speakers within a
large AS, you can configure a router to function as an IBGP route reflector. IBGP
speakers that need to communicate with other BGP speakers in the AS establish a
single peer-to-peer client session with the route reflector.
For information about the IBGP route reflector, see “Configuring IBGP as a Route
Reflector or an RR Client” on page 8-77.
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