OSPF provides support for stub areas. The concept is to reduce the number of interarea or external LSAs that get flooded into a stub area. RFC 2328 defines OSPF stub areas. RFC 1587 defines support for NSSAs. Cisco routers use totally stubby areas, such as Area 2, as shown in Figure 4-6.

Figure 4-6 OSPF Stub Networks

Stub Areas

Consider Area 1 in Figure 4-6. Its only path to the external networks is via the ABR through Area 0. All external routes are flooded to all areas in the OSPF autonomous system. You can configure an area as a stub area to prevent OSPF external LSAs (Type 5) from being flooded into that area. A single default route is injected into the stub area instead. If multiple ABRs exist in a stub area, all of them inject the default route. Traffic originating within the stub area routes to the closest ABR.

Note that network summary LSAs (Type 3) from other areas are still flooded into the stub area’s Area 1.

Totally Stubby Areas

Let’s take Area 1 in Figure 4-6 one step further. The only path to get from Area 1 to Area 0 and other areas is through the ABR. A totally stubby area does not flood network summary LSAs (Type 3). It stifles Type 4 LSAs, as well. Like regular stub areas, totally stubby areas do not flood Type 5 LSAs. A totally stubby area sends just a single LSA for the default route. If multiple ABRs exist in a totally stubby area, all ABRs inject the default route. Traffic originating within the totally stubby area routes to the closest ABR.

NSSAs

Notice that Area 2 in Figure 4-6 has an ASBR. If this area is configured as an NSSA, it generates the external LSAs (Type 7) into the OSPF system while retaining the characteristics of a stub area to the rest of the autonomous system. There are two options for the ABR. First, the ABR for Area 2 can translate the NSSA external LSAs (Type 7) to autonomous system external LSAs (Type 5) and flood the rest of the internetwork. Second, the ABR is not configured to convert the NSSA external LSAs to Type 5 external LSAs; therefore, the NSSA external LSAs remain within the NSSA.

There is also an NSSA totally stub area. The difference is that the default NSSA has no default route unless the ABR is explicitly configured to advertise one. The NSSA totally stub area does receive a default route.