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IP routing 29

8. From this database each router calculates a shortest-path tree, with

itself as root. This shortest-path tree in turn yields a routing table for

the protocol.

OSPF router types

Routers in an OSPF network can take on different roles depending their

conﬁguration. The following table describes the router types in an OSPF

network.

OSPF router types

Router Type Description

Autonomous System

Boundary Router

(ASBR)

A router attached at the edge of an OSPF network

is called an AS boundary router (ASBR). An ASBR

generally has one or more interfaces that run an

inter-domain routing protocol. In addition, any router

distributing static routes or RIP routes into OSPF is

considered an ASBR. The ASBR forwards external

routes into the OSPF domain. In this way, routers inside

the OSPF network learn about destinations outside their

domain.

Area Border Router

(ABR)

A router attached to two or more areas inside an OSPF

network is considered an area border router (ABR). ABRs

play an important role in OSPF networks by condensing

the amount of OSPF information that is disseminated.

Internal Router (IR) A router that has interfaces only within a single area

inside an OSPF network is considered an internal router

(IR). Unlike ABRs, IRs have topological information only

about the area in which they are contained.

Designated Router

(DR)

In a broadcast network a single router is elected to

be the designated router (DR) for that network. A DR

assumes the responsibility of making sure all routers on

the network are synchronized with one another and also

advertises that network to the rest of the AS.

Backup Designated

Router (BDR)

A backup designated router (BDR) is elected in addition

to the designated router (DR) and, in the event of failure

of the DR, will assume its role quickly.

OSPF host route

An OSPF router with hosts directly attached to its interfaces can use host

routes to advertise the attached hosts to its neighbors. You can conﬁgure

up to 32 host routes.

Nortel Ethernet Routing Switch 5500 Series

Conﬁguration-IP Routing Protocols

NN47200-503 03.01 Standard

5.1 27 August 2007