Reference Guide

Border Gateway Protocol IPv4 (BGPv4) | 173

Figure 9-1. BGP Autonomous Zones

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 update 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 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 routers 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. For example, as seen in the

illustration below, four routers connected in a full mesh have three peers each, six routers have 5 peers

each, and eight routers in full mesh will have seven peers each.

AS 1

AS 2

Router 2

Router 1

Router 3

Router 4

Router 7

Router 6

Router 5

Interior BGP (IBGP)

Exterior BGP (EBGP)

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