Border Gateway Protocol | 175A multihomed AS is one that maintains connections to more than one other AS. This allows the AS toremain 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 simpleexample of this is seen in Figure 9-1.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 inFigure 9-1, 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 transitservice” to the customer network, so thus the term Transit AS.When BGP operates inside an Autonomous System (AS1 or AS2 as seen in Figure 9-1), it isreferred to as Internal BGP (IBGP Interior Border Gateway Protocol). When BGP operatesbetween Autonomous Systems (AS1 and AS2), it is called External BGP (EBGP Exterior BorderGateway Protocol). IBGP provides routers inside the AS with the knowledge to reach routers external tothe AS. EBGP routers exchange information with other EBGP routers as well as IBGP routers to maintainconnectivity and accessibility.Figure 9-1. BGP Autonomous ZonesBGP version 4 (BGPv4) supports classless interdomain routing and aggregate routes and AS paths. BGPis a path vector protocol - a computer network in which BGP maintains the path that updateinformation takes as it diffuses through the network. Updates traveling through the network andreturning to the same node are easily detected and discarded.BGP does not use traditional Interior Gateway Protocol (IGP) matrix, but makes routing decisions basedon path, network policies and/or rulesets. Unlike most protocols, BGP uses TCP as its transport protocol.AS 1 AS 2Router 2Router 1Router 3Router 4Router 7Router 6Router 5Interior BGP (IBGP)Interior BGP (IBGP)Exterior BGP(EBGP)lpbgp1111
