Administrator Guide
Figure 17. Interior 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 diuses 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.
Because 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 following illustration, four
routers connected in a full mesh have three peers each, six routers have ve peers each, and eight routers in full mesh have seven
peers each.
150
Border Gateway Protocol IPv4 (BGPv4)