Concept Guide

connections from one network to another. The ISP is considered to be “selling transit service” to the customer network, so thus the
term Transit AS.
The devices within an AS (AS1 or AS2, as seen in the following illustration) exchange routing information using Internal BGP (IBGP),
whereas the devices in dierent AS communicate using External BGP (EBGP). 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. 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 diuses 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 ve peers each, and eight routers in full mesh have seven peers each.
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Border Gateway Protocol (BGP)