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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

1 2 ... 220 221 222 223 224 225 226 227 228 229 230 ... 465 466

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