12.1.4 NSSA

You can configure a non-backbone area as a not-so-stubby area (NSSA). To do so, use the area nssa configuration command.

Like a stub area, an NSSA does not receive information about external AS routes from other areas.

Even with advertised route filtering configured (using the redistribute configuration command), if you specify the no-redistribution parameter of the area nssa command, the area border router will not inject external routes into the NSSA. This reduces the amount of routing traffic in NSSAs, and minimizes the demand that route updates and route selection place on router resources.

If you execute the area nssa command with the no-summary parameter specified, the area border router does not advertise routes from other areas (inter-area routes) to the NSSA, replacing them instead with a single default route. This default route is advertised to the NSSA as inter-area routing information (a Type 3 LSA).

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(1) Advertising external AS routes

An AS boundary router in an NSSA uses Type 7 LSAs (NSSA external) to advertise external AS routes. Type 7 LSAs are propagated only within the originating area.

If you execute the area nssa command with the default-information-originate parameter specified, the area border router injects a Type 7 default route into the NSSA area. If more than one router in the NSSA advertises a Type 7 LSA for the default route, these routers select the default route with the highest precedence as an external AS route.

The area border router translates the external AS routes it learns in the NSSA to Type 5 LSAs and advertises them to other areas. The Type 5 LSA inherits the tags and forwarding address of the Type 7 LSA. If you specify the area nssa translate type7 suppress-fa configuration command in the NSSA that generates the Type 7 LSA, the router will use 0.0.0.0 as the forwarding address of the Type 5 LSA. The following figure illustrates how routes are exchanged between an NSSA and the backbone.

Figure 12-2: Route between NSSA and the backbone

(2) Restrictions

The Switch conforms to RFC 3101 (The OSPF Not-So-Stubby Area (NSSA) Option), with the exception that the following functionality is not implemented due to software limitations:

Type-7 address ranges
Type-7 translator election

As a consequence, the external AS routes learned from an NSSA are always advertised outside its boundaries.