Alcatel Carrier Internetworking Solutions 060187-10 REV D manual Stub Areas, Ospf Stub Area

Configuring OSPF OSPF Overview

Stub Areas

OSPF allows certain areas to be configured as stub areas. A stub area is an area with routers that have no AS external Link State Advertisements (LSAs).

In order to take advantage of the OSPF stub area support, default routing must be used in the stub area. This is accomplished by configuring only one of the stub area’s border routers to advertise a default route into the stub area. The default routes will match any destination that is not explicitly reachable by an intra- area or inter-area path (i.e., AS external destinations).

Backbone

		Backbone
Area 1	Area 2	Area 3
(stub)		(stub)

OSPF Stub Area

Area 1 and Area 3 could be configured as stub areas. Stub areas are configured using the OSPF ip ospf area command, described in “Creating an Area” on page 1-18. For more overview information on areas, see “OSPF Areas” on page 1-8.

The OSPF protocol ensures that all routers belonging to an area agree on whether the area has been configured as a stub. This guarantees that no confusion will arise in the flooding of AS external advertise- ments.

Two restrictions on the use of stub areas are:

•Virtual links cannot be configured through stub areas.

•AS boundary routers cannot be placed internal to stub areas.

OmniSwitch 6600 Family Advanced Routing Configuration Guide March 2005 page 1-11