The Border Gateway Protocol (BGP) is used today by routers of all Autonomous Systems (AS) in the Internet. BGP is responsible for end-to-end reachability in the Internet. BGP routers have to exchange routes towards about 200,000 prefixes (blocks of IP addresses). Inside each AS, routers are using internal iBGP sessions to exchange their best route with other routers. The objective of an iBGP topology is to redistribute BGP routes inside the whole AS. In large ASs, some BGP routers are used as route reflectors because it reduces the total number of sessions needed for this route redistribution.
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