Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Instrukcja Użytkownika Pobierz pdf (Strona 28)

Configuring IP Exterior Gateway Protocols (BGP and EGP)

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308628-15.0 Rev 00

Stub and Multihomed Autonomous Systems

An AS 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. A multihomed AS has multiple BGP

speakers. A stub AS has a single BGP speaker that establishes a peer-to-peer

session with one external BGP speaker. 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. With 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.

Some IGPs carry no BGP information. However, an OSPF type 5 LSA can carry

BGP-specific information in its tag field. Each router uses IBGP exclusively to

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

is received, it is passed 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 this service is called a transit

AS (Figure 1-3

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Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Instrukcja Użytkownika Strona 28