• multihomed AS — is one that maintains connections to more than one other AS. This group allows theAS 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. Asimple example of this group is seen in the following illustration.• stub AS — is one that is connected to only one other AS.• transit AS — is one that provides connections through itself to separate networks. For example, in thefollowing illustration, Router 1 can use Router 2 (the transit AS) to connect to Router 4. Internet serviceproviders (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 AS (AS1 or AS2, as seen in the following illustration), it is referred to as InternalBGP (IBGP Interior Border Gateway Protocol). When BGP operates between ASs (AS1 and AS2), it is calledExternal BGP (EBGP Exterior Border Gateway Protocol). IBGP provides routers inside the AS with theknowledge to reach routers external to the AS. EBGP routers exchange information with other EBGP routersas well as IBGP routers to maintain connectivity and accessibility.Figure 18. Interior BGPBGP version 4 (BGPv4) supports classless interdomain routing and aggregate routes and AS paths. BGP is apath vector protocol — a computer network in which BGP maintains the path that updated information takesas it diffuses through the network. Updates traveling through the network and returning to the same node areeasily detected and discarded.BGP does not use a traditional interior gateway protocol (IGP) matrix, but makes routing decisions based onpath, 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, a BGP network needs to be in “full mesh.” This isa topology that has every router directly connected to every other router. Each BGP router within an AS musthave iBGP sessions with all other BGP routers in the AS. For example, a BGP network within an AS needs to beBorder Gateway Protocol IPv4 (BGPv4) 192
