Why an NSSA ABR does not inject a default route automatically into NSSA unlike with stub area ?

Why an NSSA ABR does not inject a default route automatically into NSSA unlike with stub area ?

In this topology if an ASBR inject a default route into OSPF in order to permit internal ospf routers to reach another routing domain, it creates a Type-7 LSA default route and injects it into OSPF, if an NSSA ABR is designated to inject a Type-3 LSA default route automatically, an internal router in NSSA, R3 in this case will prefer a Type-3 LSA default over a Type-7 default according to the rule of OSPF path preference per RFC, as a result R3 will never reach 5.5.5.0/24, this is the reason why unlike with stub area, in NSSA area by default an ABR does not inject a default.route, giving a choice to inject a default route manually.

The routing table of R3 shown an ON2 default route (Type-7 LSA):

The ping is successfull:

Let’s inject a Type-3 LSA Default route on NSSA ABR R2:

Let’s verify the routing table of R3, indeed the Type-3 default route through R2 is installed:

The ping fails:

If an NSSA ASBR originates a  default route into the OSPF in order to reach an external domain, the default route is injected as a Type 7 NSSA External LSA.  If at the same time the NSSA ABR connecting that NSSA to Area 0 is generating a default route via a Type 3 Summary LSA, what happens?

Since Inter-Area (Type 3 LSA) MUST be preferred over NSSA External (Type 7) per the RFC, it would mean that you could never default out toward the external domain in that area because the OSPF default route via the NSSA ABR would always be preferred.  This is what would be considered “Cold Potato Routing” because you are defaulting further into your network as opposed to defaulting towards an external exit point.

RFC 3101 says in section: 1.3 Proposed Solution

The solution for this problem then is for an NSSA area, tell the NSSA ABR not auto automatically originate the default route as Type 3 LSA into the NSSA.  This gives you the choice to prefer the default route from a redistributed source, which would mean you are trying to achieve “Hot Potato Routing”. The recommended default behavior is to import summary routes into NSSA as describe in RFC 3101, see the following section below:

 

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