A BGP system consists of ASs that share network reachability information. Network reachability information is shared with adjacent BGP systems neighbors. Further logical groupings are established within BGP systems within ASs. BGP supports two types of routing information exchanges:
External BGP (EBGP) is used between ASs
EBGP speakers peer to different ASs and typically share a subnet. In an external group, the next hop is dependent upon the interface shared between the external peer and the specific neighbor. The multihop command must be specified if an EBGP peer is more than one hop away from the local router. The next hop to the peer must be configured so that the two systems can establish a BGP session.
Internal BGP (IBGP) is used within an AS
An IBGP speaker peers to the same AS and typically does not share a subnet. Neighbors do not have to be directly connected to each other. Because IBGP peers are not required to be directly connected, IBGP uses the IGP path (the IP next-hop learned from the IGP) to reach an IBGP peer for its peering connection.