BGP supports two types of exchanges of routing information: exchanges between different ASs and exchanges within a single AS. When used between ASs, BGP is called external BGP (EBGP) and BGP sessions perform inter-AS routing. When used within an AS, BGP is called internal BGP (IBGP) and BGP sessions perform intra-AS routing. Figure 8 illustrates ASs, IBGP, and EBGP.
Figure 8: ASs, External BGP, and Internal BGP
A BGP system shares network reachability information with adjacent BGP systems, which are referred to as neighbors or peers.
BGP systems are arranged into groups. In an internal BGP group, all peers in the group—called internal peers—are in the same AS. Internal peers can be anywhere in the local AS and do not have to be directly connected to each other. Internal groups use routes from an IGP to resolve forwarding addresses. They also propagate external routes among all other internal routers running internal BGP, computing the next hop by taking the BGP next hop received with the route and resolving it using information from one of the interior gateway protocols.
In an external BGP group, the peers in the group—called external peers—are in different ASs and normally share a subnet. In an external group, the next hop is computed with respect to the interface that is shared between the external peer and the local router.
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