Nortel Networks NN47200-503 Ospf normal area, 5530-24TFDconfig-router# as-boundary-router enable

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

5.127 August 2007

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