Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Manual de usuario descargar pdf (Pagina 26)

Configuring IP Exterior Gateway Protocols (BGP and EGP)

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Stub and Multihomed Autonomous Systems

An autonomous system can include one or more BGP speakers that establish

peer-to-peer sessions with BGP speakers in other autonomous systems to provide

external route information for the networks within the AS. An AS containing

multiple BGP speakers is considered to be a multihomed AS. An AS containing a

single BGP speaker that establishes a peer-to-peer session with a single external

BGP speaker is a stub AS. The BGP speaker provides external route information

only for the networks contained within its own AS.

Interior BGP Routing

Nortel Networks implements Interior BGP (IBGP) intra-AS routing. Under IBGP,

each router in the AS runs an interior gateway protocol (IGP), such as OSPF, for

internal routing updates and also maintains an IBGP connection to each BGP

border router. The IBGP information, along with the IGP route to the originating

BGP border router, determines the next hop to use for external networks.

The IGP carries no BGP information. Each router uses IBGP exclusively to

determine reachability to external networks. When an IBGP update for a network

is received, it can be passed on to IP for inclusion in the routing table only if a

viable IGP route to the correct border gateway is available.

An AS with more than one BGP speaker can use IBGP to provide a transit service

for networks outside the AS. An AS that provides such a service for BGP speakers

is known as a transit AS (see Figure 1-3

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Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Manual de usuario Pagina 26