Dell S6000 FTOS 9.0(2.0) Configuration Guide for the System - Page 139

When BGP operates inside an Autonomous System (AS1 or AS2 as seen in Figure 8-1), it is referred to as Internal BGP (IBGP Interior Border Gateway Protocol). When BGP operates between Autonomous Systems (AS1 and AS2), it is called External BGP (EBGP Exterior Border Gateway Protocol). 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 8-1. BGP Autonomous Zones lpbgp1111 Router 3 Router 5 Router 1 Router 2 Router 4 Exterior BGP (EBGP) Router 7 Router 6 AS 1 Interior BGP (IBGP) AS 2 Interior BGP (IBGP) 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 Figure 8-2, 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. Border Gateway Protocol | 117