Quick question regarding NSSA. If I read correctly, NSSA allows type 7 LSA, so therefore It can allow redistributed routes into it. It then turns LSA 7 into an LSA 5 to send out.
Lets say NSSA is connected to Area 0. And Area 0 has some redistributed routes into it. In this scenario I think the LSA type will be 5 into the NSSA area and then it won't be allowed?
Maybe I have some confusion.
Thanks for the clarification. I think that I understand your question better now. To help you understand better I think that it would be good to consider the link state database(s) of several routers in your example. In area 0 ASBR there is a single link state data base. As EIGRP routes are redistributed into OSPF the ASBR creates type 5 LSAs in its data base and advertises those LSAs to other routers in area 0 including the ABR with area 1. On the ABR there are two separate link state data bases. There is a link state data base for area 0 and a link state data base for area 1. As the ABR receives updates from the ASBR it puts type 5 LSAs into its area 0 link state data base. The ABR does not put the type 5 LSAs into the area 1 data base because you do not advertise External routes into an NSSA. Inside area 1 there is another ASBR which is redistributing RIP. This ASBR has only a single link state data base. As RIP routes are redistributed it puts type 7 LSAs into its link state data base and advertises them to the ABR. As the ABR receives the type 7 LSAs from the area 1 ASBR it puts type 7 LSAs into its area 1 link state data base. And the ABR creeates type 5 LSAs from the type 7s and puts the type 5 LSAs into its area 0 link state data base and advertises these routes into area 0.
So what happens is that RIP routes are redistributed into area 1 NSSA as type 7 and they are translated into type 5 and advertised to area 0 routers. EIGRP routes are redistriburted into area 0 as type 5 and are not advertised into area 1.
This functionality is one of the things that you must consider as you are determining whether to configure some areas as NSSA - the routers in NSSA will have visibility into some externals but will not have visibility into some other externals. If that works for your design then NSSA is an option for you. If that does not work for your design then NSSA is not a good option for you.
NSSA is simply a stub area that allows for redistribution via an ASBR inside the NSSA area, in NSSA there is NO Type 5 external and no Default route, only Type 3 and Type 7 (instead of Type 5 or the default route, uses the local ASBR for external routes using Type 7). The ABR doesn't originate a default route to the NSSA routers by default. If a default route is forced manually (it will be a Type 7 LSA as it is the only external route allowed inside the NSSA area), more over the ABR of the NSSA converts the Type7 LSAs to Type5 before advertising it into area0.