Switch User Manual

IP routing conﬁguration examples 139

The impact of a topology change is localized to the area in which it occurs.

The only exception to this is for the area border routers, which must

maintain an LSDB for each area to which they belong. Changes in topology

are advertised to the rest of the network by the area border routers by

advertising summary LSAs.

A 32-bit Area ID, expressed in IP address format such as 0.0.0.0 for

0 identiﬁes areas. Area 0 is also known as the backbone area and is

responsible for distributing routing information to all other areas.

If multiple areas are used, they must all be attached to the backbone

through an Area Border Router (ABR), which connects area 0.0.0.0 to the

non-backbone areas.

Normal area A normal area is a collection of routers that use the same

Area-ID that calculates inter-area and external routes through the use of the

following link state advertisements:

•

Summary LSAs

•

ASBR-summary LSAs

•

AS-external LSAs

As illustrated in the following ﬁgure, a normal area supports Area Border

Routers (ABRs) and Autonomous System Border Routers (ASBRs).

OSPF normal area

The Nortel Ethernet Routing Switch 5500 Series automatically becomes an

ABR when more than one area is conﬁgured for it and each area has at

least one operational interface. To designate a router as an ASBR, use the

as-boundary-router enable command.

5530-24TFD(config-router)# as-boundary-router enable

Nortel Ethernet Routing Switch 5500 Series

Conﬁguration-IP Routing Protocols

NN47200-503 03.01 Standard

5.1 27 August 2007