Confederations
A confederation[2] is another way to deal with the explosion of an IBGP mesh within an AS. As with route reflection, confederations are recommended only for cases in which IBGP peering involves a large number of IBGP peering sessions per router.
Tip
See the section Confederations in Chapter 12.
BGP confederations are based on the concept that an AS can be broken into multiple sub-ASs. Inside each sub-AS, all the rules of IBGP apply. All BGP routers inside the sub-AS, for example, must be fully meshed. Because each sub-AS has a different AS number, external BGP must run between them. Although EBGP is used between sub-ASs, routing inside the confederation behaves like IBGP routing in a single AS. In other words, the next hop, ...
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