
Exterior Gateway Protocols (BGP and EGP)
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Figure 1-3. Transit AS
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.
AS 20
AS 10
AS 50
AS 11
AS 12
AS 30
IGP
BGP A
BGP B
BGP C
IP0021A
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