Well, not quite. It is possible to have more than one ABR in all types of stub areas. Stub areas are defined by what types of LSAs their ABRs filter out. If you filter them appropriately at the ABRs, then you can achieve the type of stub area you want.
Also, both the NSSA and the Totally NSSA areas allow ASBRs. The difference is that Totally NSSAs don’t allow Type 3 LSAs. That simply means that a totally NSSA area will not learn about the summary routes from other non-backbone areas. Take a look at the OSPF LSA Types Explained lesson for more info about that.
So to summarize,
NSSA - blocks all Type 5 external LSAs but allows an ASBR
Totally NSSA - block all Type 5 external LSAs, and type 3 summary LSAs from other areas, but allows an ASBR.
You have mentioned ‘If you configure an area as a stub it will block all type 5 external LSAs. All the prefixes you redistributed into OSPF from another routing protocol are not welcome in the stub area. Since you are not allowed to have type 5 external LSAs in the stub area, it’s also impossible to have an ASBR in the stub area. To reach networks in other areas, there will be a default route.’
Everything makes sense except for the part ‘To reach networks in other areas, there will be a default route’. According to my understanding, only type 5 external LSAs in the stub area are not allowed, Type 3 are still allowed. So Default route is only for external domains. Not for other areas!
You are correct in that in a stub area, a type 3 LSA will still be allowed, and an OSPF router within a stub area will see specific routes advertised via Type 3 LSAs. For type 5 LSAs, the configuation of a stub area will cause all routes learned via type 5 LSAs to be replaced with a default route and that is what Rene was pointing out in the text you shared. So even if you have Type 3 LSAs, you will still have the addition of a default route replacing all type 5 LSAs. You can see this in action in the following lesson:
I will let Rene know to take a look at the text and make any necessary clarifications.
For NSSA and totally NSSA they do not accept type 5 LSAs however if they come from an ASBR instead of an ABR then they become type 7 LSAs so then if an asbr using rip was redistributed into an ospf NSSA it could learn those routes but if it came from an ABR trying to redistribute rip into that NSSA area it would learn it because it would be type 5. Am i thinking of this correctly?
Yes, if an ASBR redistributes RIP routes into an NSSA or a totally NSSA, then those routes can be learned by the rest of the OSPF topology.
Not quite. Because if an ABR tries to redistribute RIP into an NSSA, then it is by definition an ASBR. So it would use type 7 and it would function correctly. Does that make sense?