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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.
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 five peers each, and eight routers in
full mesh have seven peers each.
Figure 17. BGP Routers in Full Mesh
The number of BGP speakers each BGP peer must maintain increases exponentially. Network management quickly becomes
impossible.
Border Gateway Protocol IPv4 (BGPv4)
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