NSSA and redistribution

Unanswered Question
Sep 27th, 2007

Hi, i'm just reading through some things about NSSA and redistribution and i came across this link on the cisco site...


it states that;

"Redistributed RIP routes are not allowed in Area 1 because NSSA is an extension to the stub area. The stub area characteristics still exist, which includes no type 5 LSAs allowed."

I don't really understand that statement..i dont get why it says the IGRP can be injected into the area but RIP can't... can someone explain that to me please?


I have this problem too.
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ericpiros Thu, 09/27/2007 - 22:05

With reference to the first diagram, IGRP routes will be allowed inside Area 1 since it is an NSSA area and the IGRP network is directly connected to Area 1. So IGRP will be distributed inside Area 1 as Type 7 LSAs. The Area 1 ABR will distribute these Type 7 LSAs to Area 0 as Type 5 LSAs.

Now if we assume that Area 2 is also NSSA, the same thing will happen. RIP routes will be distributed inside Area 2 as Type 7 LSAs and as Type 5 LSAs into area 0.

So Area 0 will know about the RIP and IGRP routes. Now since Area 1 and 2 are NSSA areas, they will block Type 5 LSA just like Stub areas. So Area 1 will not know about the RIP routes because it will filter Type 5 LSA. Area 2 will not know about the IGRP routes since they will also be filtering Type 5 LSAs.

LucaSalvatore_2 Thu, 09/27/2007 - 22:24

Ok thanks for the reply... so let me just see if i have this straight:

the IGRP routes will be allowed into area 1 because the type 5 LSAs will get converted into type 7 LSAs at the NSSA ASBR.

the rip routes will not be allowed into area 1 because they are type 5 LSAs which wont hit a ASBR which is needed to convert them to type 7.

is that on the right track?

ericpiros Sun, 09/30/2007 - 20:09

The IGRP routes will be allowed into area 1 since the IGRP external routes are directly connected to area 1. So the ASBR there inserts the IGRP routes as Type 7 LSAs. The RIP routes will not be allowed into area 1 because these routes are not directly connected to area 1 (they are connected to area 2). For the RIP routes to go to area 1 they are converted to Type 5 LSAs then sent to Area 0. However since NSSA areas do not accept Type 5 LSAs, they will not appear on area 1 routers.

LordFlasheart Fri, 10/12/2007 - 01:15


NSSAs are just like stub areas but allow ASBRs in them. As there is an ASBR between the IGRP and OSPF network at the NSSA then the IGRP routes can be redistributed as Type 7 LSAs.

If you look at the RIP routes it eventually hits a NSSA ABR, not an ASBR, so it will not forward the default Type 5 LSAs into the NSSA as it is not allowed.




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