Dell S4048–ON Configuration Manual page 177

IBGP provides routers inside the AS with 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. 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.
Border Gateway Protocol IPv4 (BGPv4) 177