If a router has a BGP session to a neighbor with a different AS number, the session is called external BGP (EBGP) if the neighbor has the same AS number as the router, the session is called internal BGP (IBGP). The AS numbers on the AS_PATH list are used for loop detection a router receiving a BGP route with its own AS number in the AS_PATH assumes a loop and discards the route. Given multiple routes to the same destination, the route with an AS_PATH listing the fewest AS numbers is assumed to be the shortest path. The AS_PATH attribute is a shortest path determinant.
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Unique among the common IP routing protocols, BGP sends only unicast messages and forms a separate point-to-point connection with each of its peers.īGP is an application layer protocol using TCP (port 179) for this point-to-point connection and relies on the inherent properties of TCP for session maintenance functions such as acknowledgment, retransmission, and sequencing.īGP is a vector protocol, although called a path vector rather than distance vector because it sees the route to a destination as a path through a series of autonomous systems rather than as a series of routers hops.Ī BGP route describes the path vector using a route attribute called the AS_PATH, which sequentially lists the autonomous system numbers comprising the path to the destination. The section “BGP Basics” in Chapter 1 introduced you to the fundamental facts about BGP.
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