Stub and Not-So-Stubby Areas
Another way to reduce the route information advertised is to make an area a
stub area. Configuring an area as a stub area forces its ABR to drop all
external (type 5) routes and replaces them with a default route. To limit
routing information even more, an area can be made totally stubby using the
no-summary keyword on the ABR only. In that case, all interarea and external
routes are dropped by the ABR and replaced by a default route. The
default route starts with a cost of 1; to change it, use the area default-cost
command. The example that follows shows area 2 configured as a totally
stubby area, and the default route injected with a cost of 5:
Router(config-router)#area 2 stub no-summary
Router(config-router)#area 2 default-cost 5
Stub areas are attractive because of their low overhead. They do have some
limitations, including the following:
�¡ Stub areas can�ft include a virtual link.
�¡ Stub areas can�ft include an ASBR.
�¡ Stubbiness must be configured on all routers in the area.
Another kind of stub area is a not-so-stubby area (NSSA). NSSA is like a
stub or totally stub area, but allows an ASBR within the area. External
routes are advertised as type 7 routes by the ASBR. The ABR converts them
to type 5 external routes when it advertises them into adjacent areas. NSSA
is configured with the area nssa command under the OSPF routing process.
The no-summary keyword on the ABR configures the area as a totally
NSSA area; this is a Cisco proprietary feature. By default, the ABR does not
inject a default route back into an NSSA area. Use the default-informationoriginate
keyword on the ABR or ABR to create this route.
Router(config-router)#area 7 nssa [no-summary] [defaultinformation-
originate]