Figure 21. Internal BGP
BGP version 4 (BGPv4) supports classless interdomain routing and aggregate routes and AS paths. BGP is
a path vector protocol — a computer network in which BGP maintains the path that updated information
takes as it diffuses through the network. Updates traveling through the network and returning to the
same node are easily detected and discarded.
BGP does not use a traditional interior gateway protocol (IGP) matrix, but makes routing decisions based
on path, network policies, and/or rulesets. Unlike most protocols, BGP uses TCP as its transport protocol.
Since each BGP router talking to another router is a session, a BGP network needs to be in “full mesh.”
This is a topology that has every router directly connected to every other router. Each BGP router within
an AS must have iBGP sessions with all other BGP routers in the AS. For example, a BGP network within
an AS needs to be in “full mesh.” As seen in the illustration below, four routers connected in a full mesh
have three peers each, six routers have five peers each, and eight routers in full mesh have seven peers
each.
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Border Gateway Protocol IPv4 (BGPv4)
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