Can an external default route be injected into a NSSA totally stub area?

Question

According to what I learned from Cisco documents, by default a normal NSSA area's ABR does not create a default route. Unless you use the command:

Area 1 nssa default-information-originate

on ASBRs or ABRs.But if an area is a totally NSSA area, its ABR will create a Type 3 LSA containing a default route in the area.

Does this mean if an area is NSSA totally, the ASBR's external default route would conflict with ABR's default route? I know I should test it in the lab first, but I cannot help getting the answer immediately.

Thanks for your answers!

Answer 1

You are confusing a couple of terms. An NSSA (Not So Stubby Area) area is not a Totally Stubby Area. The difference is actually rather large. An NSSA area can contain an ASBR, but a Totally Stubby Area cannot. An NSSA area allows routes from outside the AS to be introduced into the OSPF AS, but a Totally Stubby Area does not. These are types are mutually incompatible.

Edit:

Based on your edit, you need to understand that the no-summary keyword on an ABR will suppress any regular Type-3 and Type-4 LSAs, and only send in a default route into the area. A default route from an ASBR, or from the area itself, will then have no meaning.

The default-information-originate keyword injects a default route into OSPF from an area if the area has a default route in its route table. This is a route leaving the area, compared to one entering the area. What should a router do about a default route from within the area compared to one from a different AS? OSPF trusts itself more than an external route, so the external default route is suppressed. This can be modified by changing the metric types and costs.

Answer 2

If the ASBR at the edge of an NSSA receives- and redistributes- a default route then it will show up within the area as an N1 or N2. This is basically the type-7 equivalent of the usual type-5 E1/E2 external routes in a standard area.

A default route originated by the ABR with the command you listed will show up within the NSSA as an inter-area summary route (i.e. type-3).

To the various routers within the NSSA a type-3 is going to be preferred over an external route. This is just the basic behavior of route selection in OSPF (O > O IA > E1 > E2 > N1 > N2).