OSPF NSSA

Answered Question
Aug 18th, 2007
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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.

Correct Answer by Richard Burts about 9 years 11 months ago

Ross


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.


HTH


Rick

Correct Answer by mohammedmahmoud about 9 years 11 months ago

Hi,


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.



HTH,

Mohammed Mahmoud.

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Richard Burts Sat, 08/18/2007 - 17:14
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Ross


Perhaps I do not understand your post. I am not clear in your discussion whether there is an area NSSA and also area 0 or whether you are suggesting that area 0 is NSSA.


But area 0 can not be NSSA. Any area that is NSSA must be non-zero area. Area 0 can not be stub, can not be totally stub, and can not be NSSA.


HTH


Rick

rossmillan Sat, 08/18/2007 - 22:24
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Sorry its hard to explain whilst I am trying to get my head around it. Lets say we have:


EIGRP->OSPF Area 0<->Ospf area 1(nssa)<-> RIP


So in my example RIP will be redistributed into OSPF area 1, using a LSA type 7. The area 1 router turns it into lsa type 5 when propogating it to Area 0 correct?


But if EIGRP is redistributed into Area 0. Then area 0 converts it to LSA type 5 into area 1 correct? IN which case it must only get a default route and not a specific EIGRP route correct?


I guess then my question is that the NSSA will not accept LSA type 5 from EIGRP, yet it turns LSA type 7 into LSA type 5 for rip, and that works?


Very confused.

Correct Answer
mohammedmahmoud Sat, 08/18/2007 - 23:27
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Hi,


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.



HTH,

Mohammed Mahmoud.

Correct Answer
Richard Burts Sun, 08/19/2007 - 04:10
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Ross


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.


HTH


Rick

rossmillan Sun, 08/19/2007 - 15:29
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Thanks for such a detailed explanation. Until now I hadn't really thought of them as seperate link state data bases. That is a good and clear distinction, which makes it easier for me to understand.


Many thanks

Richard Burts Sun, 08/19/2007 - 18:48
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Ross


I am glad that my explanation helped you to understand this concept better (and thanks for the rating). It is one of the fundamental (but frequently not well understood) concepts of OSPF that a router maintains a separate link state data base for each of the OSPF areas in which it participates.


HTH


Rick

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