Concept Guide

the knowledge to reach routers external to the AS. EBGP routers exchange information with other EBGP routers as well as
IBGP routers to maintain connectivity and accessibility.
Figure 17. BGP Topology with autonomous systems (AS)
BGP version 4 (BGPv4) supports classless interdomain routing (CIDR) 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, an IBGP 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 (BGP)