Not-so-stubby-area (NSSA)

This area type connects to the backbone area through one or more ABRs. NSSAs are used where an ASBR exists in an area where you want to:

  • Block injection of external routes from other areas of the AS.

  • Advertise type-7-LSA external routes (learned from the ASBR) to the backbone area as AS-external-LSAs.

NSSAs also support the following:

  • Advertise inter-area-prefix-LSAs from the backbone area into the NSSA. (If no-summary is enabled, the NSSA ABR suppresses these LSAs from the backbone and, instead, injects the inter-area-prefix-LSA default route into the NSSA.)

  • Advertise NSSA inter-area-prefix-LSAs to the backbone area.

In the above operation, the ASBR in the NSSA injects external routes as type-7-LSAs. (AS-external-LSAs are not allowed in an NSSA.) The ABR connecting the NSSA to the backbone converts the type-7-LSAs to AS-external-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 inter-area-prefix-LSAs from the backbone area into the NSSA.

The default route (::/0) is always injected into the NSSA as either a type-7-LSA or an inter-area-LSA, depending on the no-summary configuration (default: disabled). That is, if inter-area-prefix-LSAs are allowed in the NSSA (the default operation), a type-7-LSA default route (::/0) is injected into the NSSA. But if inter-area-prefix-LSAs are blocked (by enabling no-summary), the inter-area-prefix-LSA default route is injected into the NSSA instead of the type-7-LSA default route.

You can also configure the NSSA ABR to suppress advertising some or all of the area's summarized internal or external routes into the backbone area. See router ospf3 area.

Virtual links are not allowed for NSSAs.