stub vs nssa ospf If I want to advertise an OSPF default into an NSSA, why does the area need explicit configuration to advertise a default route: * A Totally Stubby NSSA (advertise a Type 3 summary LSA default route via the `no-summary keyword` on the ABR) * The ABR / ASBR (advertise a Type 7 LSA default route - `default-information-originate keyword` on the ABR) This is in contrast with the a stub area where the ABR advertises the Type 3 summary LSA default route without extra config.

The reason you need to explicitly inject a default route into a NSSA area is because the external route(s) you are injecting could also be (and often is) a default route. If you redistribute a default route (for example, a static route) into an NSSA, that would conflict with the type 3 default route from the ABR. To avoid that problem, the type 3 default route is optional.