Figure 41: A Fully Meshed Autonomous System
Figure 42 on page 146 illustrates how you can create three sub-ASs within AS 29 to
greatly reduce the number of peering sessions. According to common practice, use
a number from the private range of AS numbers—from 64512 to 65535—to identify
each sub-AS. AS 29 is now a confederation of three sub-ASs: AS 64720, AS 64721,
and AS 64722. Each sub-AS consists of fully meshed IBGP peers. A slightly modified
version of EBGP runs between the sub-ASs: It acts like IBGP within an AS because
the local-pref, MED, and next-hop attributes are preserved across the sub-AS
boundaries. To the external neighbors, AS 29 appears the same as it ever was.
Managing a Large-Scale AS ■ 145
Chapter 1: Configuring BGP Routing
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