When two BGP speakers are in the same autonomous system, the BGP session is called an internal BGP session, or IBGP session. When two BGP speakers are in different autonomous systems, the BGP session is called an external BGP session, or EBGP session. BGP uses the same types of message on IBGP and EBGP sessions, but the rules for when to send which message and how to interpret each message differ slightly; for this reason some people refer to IBGP and EBGP as two separate protocols.
IBGP requires that BGP speakers within an autonomous system be fully meshed, meaning that there must be a BGP session between each pair of peers within the AS. IBGP does not require that all the peers be physically connected. EBGP does not require full meshing of BGP speakers. EBGP sessions typically exist between peers that are physically connected.
Figure 2 shows an example of the exchange of information between routers running IBGP and EBGP across multiple ASs.
Figure 2: Internal and External BGP
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