Not-so-stubby-area (NSSA)

This area is available and connects to the backbone area through one or more ABRs. NSSAs are intended for use where an ASBR exists in an area where you want to control the following:

Advertising the ASBR's external route paths to the backbone area
Advertising the NSSA's summary routes to the backbone area
Allowing LSAs from the backbone area to advertise in the NSSA:
- Summary routes (type-3 LSAs) from other areas
- External routes (type-5 LSAs) from other areas as a default external route (type-7 LSAs)

In the above operation, the ASBR in the NSSA injects external routes as type 7 LSAs. (Type 5 LSAs are not allowed in an NSSA.) The ABR connecting the NSSA to the backbone converts the type 7 LSAs to type 5 LSAs and injects them into the backbone area for propagation to networks in the backbone and to any normal areas configured in the AS. The ABR also injects type-3 summary LSAs:

From the NSSA into the backbone area
From the backbone into the NSSA

If the ABR detects type-5 external LSAs on the backbone, it injects a corresponding type-7 LSA default route (0.0.0.0/0) into the NSSA

You can also configure the NSSA ABR to do the following:

Suppress advertising some or all of the area's summarized internal or external routes into the backbone area. See Configuring ranges on an ABR to reduce advertising to the backbone.
Replace all type-3 summary routes and the type-7 default route with the type-3 default summary route (0.0.0.0/0.)

Virtual links are not allowed for NSSAs.