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 keywordon the ABR) - The ABR / ASBR (advertise a Type 7 LSA default route –
default-information-originate keywordon 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.
Best Answer
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.