Dell Force10 S4820T S4820T Configuration Guide - Page 181

A multihomed AS is one that maintains connections to more than one other AS. This allows the AS to remain connected to the internet in the event of a complete failure of one of their connections. However, this type of AS does not allow traffic from one AS to pass through on its way to another AS. A simple example of this is seen in the illustration below. A stub AS is one that is connected to only one other AS. A transit AS is one that provides connections through itself to separate networks. For example as seen in the illustration below, Router 1 can use Router 2 (the transit AS) to connect to Router 4. ISPs are always transit ASs, because they provide 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. When BGP operates inside an Autonomous System (AS1 or AS2 as seen in the illustration below), 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. 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 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 will have seven peers each. Border Gateway Protocol IPv4 (BGPv4) | 181