OSPF NSSA Question?

Question

Why do the NSSA link state advertisements show up as type 5 in Area 0? How and why is the conversion done?

Answer 1

NSSA is an extension of the standard stub area feature that allows external routes to be injected. When you redistribute into an NSSA area it creates the special type 7 LSA. Say if you redistribute from RIP into an OSPF NSSA, then the RIP networks will come through the ASBR as a type 7 LSA. So far, so good. Now, when that LSA crosses the NSSA ABR into Area 0, the ABR translates it into a type 5 LSA when it crosses into Area 0. Suppose it's area 100, then instead of using an "area 100 stub" command it's the "area 100 nssa" command. The ABR will also summarize the redistributed routes from the non-OSPF network. Also you can use a default-information-originate option on the "area 100 nssa" command on the ABR to cause it to generate a default route.

Answer 2

Ditto what he said, but a bit more.

Type 5 LSAs aren't sent into a stub area, but since you have an ASBR in the NSSA, you have to do *something* to get the external summaries through. So, type 7 was created. Once the type 7 hits the ABR between the NSSA and area 0, it is converted to a type 5 for flooding to the rest of the network.