Routes advertised by BGP maintain a list of the ASs through which the route travels. This information is stored in the route advertisement as the AS path, and it is one of the primary criteria that a local router uses to evaluate BGP routes for inclusion in its forwarding table. Figure 59 shows how BGP creates an AS path.
Figure 59: BGP AS Path
In the network shown in Figure 59, the route from Host A to Host B travels through two intermediate ASs. As the route advertisement is propagated through the BGP network, it accumulates an AS path number each time it exits one AS and enters another. Each AS number is prepended to the AS path, which is stored as part of the route advertisement. When the route advertisement first leaves Host B's AS, the AS path is 17. When the route is advertised between intermediate ASs, the AS number 7 is prepended to the AS path, which becomes 7 17. When the route advertisement exits the third AS, the AS path becomes 4 7 17. The route with the shortest AS path is preferred for inclusion into the BGP forwarding table.