3

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.

4

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.

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