BGP was originally intended to run along the borders of an AS with the routers in the middle of the AS ignorant of the details of BGP (hence the name "Border Gateway Protocol"). A transit AS, such as the one in the figure, is an AS that routes traffic from one external AS to another external AS. Typically, transit autonomous systems are Internet service providers (ISPs). All routers in a transit AS must have complete knowledge of external routes. Theoretically, one way to achieve this goal is to redistribute BGP routes into an IGP at the edge routers. However, this approach has problems.
Because the current Internet routing table is very large, redistributing all the BGP routes into an IGP is not a scalable method for the interior routers within an AS to learn about the external networks. Another method that you can use is to run IBGP within the AS.
© 2006 Cisco Systems, Inc.
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