The Internet is viewed as a set of arbitrarily connected AS's. Routers that communicate directly with each other via BGP are known as BGP speakers. BGP speakers can be located within the same AS or in different AS's. BGP speakers in each AS communicate with each other to exchange network reachability information based on a set of policies established within each AS. For a given BGP speaker, some other BGP speaker with which the given speaker communicates is referred to as an external peer if the other speaker is in a different AS, while if the other speaker is in the same AS it is referred to as an internal peer.
There can be as many BGP speakers as deemed necessary within an AS. Usually, if an AS has multiple connections to other AS's, multiple BGP speakers are needed. All BGP speakers representing the same AS must give a consistent image of the AS to the outside. This requires that the BGP speakers have consistent routing information among them. These gateways can communicate with each other via BGP or by other means. The policy constraints applied to all BGP speakers within an AS must be consistent. Techniques such as using a tagged IGP (see A.2.2) may be employed to detect possible inconsistencies.
In the case of external peers, the peers must belong to different AS's, but share a common Data Link subnetwork. This common subnetwork should be used to carry the BGP messages between them. The use of BGP across an intervening AS invalidates the AS path information. An Autonomous System number must be used with BGP to specify which Autonomous System the BGP speaker belongs to.