
Configuring BGP Services
5-21
Using IBGP in a Transit AS
If an AS has more than one BGP speaker, it can provide transit service between
multiple networks outside the AS. An AS that provides such for a service for BGP
speakers is known as a transit AS (see Figure 5-4).
Figure 5-4. Transit Autonomous System
It is important that there be a consistent view of routing within the transit AS. This
view is provided by whichever IGP the AS is running. It is also important that
routes exterior to the AS be consistent. This can be accomplished by having all of
the BGP speakers within the AS that connect to exterior ASs maintain direct
connections with each other. This is known as internal BGP (IBGP). The speakers
then agree upon which border routers will serve as exit/entry points for particular
networks outside the AS. All internal routers must be updated with this transit
information before transit service is advertised to other ASs.
In Figure 5-4, Autonomous System 20 is the transit AS. It is providing
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.
AS 20
AS 10 AS 50
AS 11
AS 12
AS 30
IGP
BGP A
BGP B
BGP C
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