:A neighbor connection (also referred to as a peer connection) between two routers can be established within the same AS, in which case BGP is called internal BGP (IBGP). Likewise, a peer connection between routers in different ASs is referred to as external BGP (EBGP).
:The neighbor negotiation process is mainly the same for internal and external neighbors as far as building the TCP connection at the transport level.
:Neighbors can reach one another via some Interior Gateway Protocol (IGP), the BGP session is established, and BGP messages are exchanged.
:Generally, for external BGP sessions, a route through a directly connected interface establishes IP reachability. Indirectly connected external neighbors require extra configuration.
:A BGP session formed between external BGP peers that are not physically connected is referred to as multihop EBGP.
:The current authentication features available in BGP-4 use the message-digest version 5 (MD5) algorithm.
:it is important to maintain a full IBGP mesh within the AS.
:By definition, the default behavior of BGP requires that it must be synchronized with the IGP before BGP may advertise transit routes to external ASs. The consequence of injecting BGP routes inside an IGP is costly. Redistributing routes from BGP into the IGP will result in major overhead on the internal routers, primarily from an IGP scalability perspective, because (as discussed earlier) IGPs are not designed to handle that many routes. That said, by far the most common configuration in Internet-connected networks is to disable BGP synchronization and rely on a full mesh of IBGP routers.
reference :: Internet Routing Architecture
