RouterGeek

This article is a continuation of the previous article on Configuration of Mutiple-Area OSPF on Cisco Routers. Refer to the previous article for the essential configuration of Multiple-Area OSPF.

1. ASBR

A router is called Autonomous System Boundary Router (ASBR) when it connects the OSPF area to a different autonomous system. This router should be configured with a summary-address to summerize routes received into OSPF via redistribution:

Router(config-router)#summary-address SUM.MAR.RYA.DRS SUB.NET.MAS.K00

where

SUM.MAR.RYA.DRS is the summary-address for the summerized subnets

SUB.NET.MAS.K00 is the subnet mask of the summarized address.

This command is usually issued at the router connecting the internetwork to the Internet.

2. Stub Areas

Stub areas in OSPF are areas where only one ABR is there, or where co-located ABRs exist. For this kind of areas, the following configuration can be made to reduce the routing traffic between the ABRs:

Router(config-router)#area X stub

where X is the area number.

This command should be issued on both ABR; the stub area ABR and the Area 0 ABR that is connected to the stub area. If more than one router exists in the stub area, the previous command need to be issued on ALL routers of the stub area.

Usually, if there is only one router in the stub area, instead of defining all directly-connected networks, the following command is used:

Router(config-router)#network 0.0.0.0 255.255.255.255 area X

where X is the area number.

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Read the rest of the article in the Router Geek Book: Guide to Cisco Routers Configuration