How to configure OSPF NSSA (Not So Stubby) Area

[lagapidis (Lazarus Agapidis)]

Hello Leonardo

@leonardograzzi841

Yes, I don’t understand why the default route is not originated by default on the ABR as in a stubby area, because I cannot think of a case when the ASBR is the one generating the default route, because it would be redundant to the Type 7 LSAs generated by the ASBR, is that correct?

The logic of a stub area is that you have only one possible route out of the area. Therefore, why not replace all routing information with a single default route to the ABR? That gives us more efficient routing tables within the stub. If you have an ASBR however, there are now two possible exit points from the area, via the ABR to the rest of the OSPF domain, or out of the ASBR to a routing domain external to OSPF. Regardless of the LSA types allowed or not, the automatic default route generation is left up to the network engineer.

@leonardograzzi841

…so then OSPF “waits” for us to decide which router would be the one generating the default route. Is this correct?

Yes, your understanding makes sense. Although not explicitly stated in the RFC 3101 which describes NSSAs, the RFC does indicate that ABRs do not automatically generate default routes (Type-3 LSAs for 0.0.0.0/0) into NSSA areas unless explicitly configured to do so. This design choice ensures that NSSA areas maintain control over routing information and avoid unintended routing behavior.

I hope this has been helpful!

Laz