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
When summary routes are not imported into an NSSA, the default LSA
originated into it by its border routers must be a Type-3 summary-
LSA. This default summary-LSA insures intra-AS connectivity to the
rest of the OSPF domain, as its default summary route is preferred
over the default route of a Type-7 default LSA. Without a default
summary route the OSPF domain's inter-area traffic, which is normally
forwarded by summary routes, might exit the AS via the default route
of a Type-7 default LSA originated by an NSSA internal router. The
Type-7 default LSAs originated by NSSA internal routers and the no-
summary option are mutually exclusive features. When summary routes
are imported into the NSSA, the default LSA originated by a NSSA
border router into the NSSA should be a Type-7 LSA.
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”.
