BGP is the principal interdomain routing protocol of the Internet. It advertises reachable IP prefixes together with attributes that help networks select and propagate routes.

A central attribute is the AS path: the sequence of autonomous systems through which an advertisement has passed. If an AS sees itself already in that path, it can reject the route and thereby prevent simple inter-AS loops.

BGP route selection is policy driven, not pure shortest-path routing. Operators can prefer customer routes, alter local preference, filter advertisements or choose among several paths according to administrative goals.

For example, two routes may reach the same prefix through three and four ASes, yet local policy can deliberately prefer the four-AS path.

BGP therefore combines distributed reachability with explicit policy. Its behavior reflects both network topology and relationships between independently operated networks.