Need clarification on ospf nssa area

[gopal_4476]

Dear friends,

Can any one tell me the diffrence between the NSSA area and the other areas in ospf domain and nssa genreates type 7 LSA right... what messaged type 7 LSA carries. Need clarification on this.

Regards,

Gopal

[pkhatri]

Gopal,

The major difference between an NSSA area and a 'normal' stub area is that you can inject external routes into NSSA areas. In non-stub areas, external routes appear as type-5 LSAs. In NSSAs, external routes appear as type-7 LSAs which are then translated to type-5 LSAs for distribution throughout the rest of the OSPF domain.

In terms of content, the type-7 LSAs are almost identical to type-5 LSAs...

Hope that helps - pls rate the post if it does.

Paresh

[balajitvk]

Hi Gopal,

It is a non-proprietary extension of the existing stub area feature that allows the injection of external routes in a limited fashion into the stub area.

It is very much like stub area but it carries routers from other AS.

Other area or other AS routes in NSSA is called LSA7.Redistribution into an NSSA area creates a special type of link-state advertisement (LSA) known as type 7, which can only exist in an NSSA area. An NSSA autonomous system boundary router (ASBR) generates this LSA and an NSSA area border router (ABR) translates it into a type 5 LSA to other area.

For more info,

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a88.shtml#topic1

Rate if it helps,

Rgs.

[ankurbhasin]

Hi Balaji,

Thanks a lot for posting this link.

Long time back I was having this link and few days back when I tried to open this link I was not able to open it as cisco might have shifted this link from cco

http://www.cisco.com/en/US/partner/tech/tk365/tk480/technologies_tech_note09186a0080094a88.shtml

That was my link which is not opening now for me atleast.

But thanks for giving me the new URL for the same matter.

You get bingo 5 from my side.

Regards,

Ankur

[gopal_4476]

Friends,

in NSSA ASBR genrates type 7 LSA right but wht about type 5 LSA its also genrated by ASBR???

Regards,

Gopal

[pkhatri]

The type-5 will be generated by the NSSA ABR (which could also be an ASBR, but does not need to be one). If there are more than one ABRs in the area, one of them will be designated as the translator for the area and will translate all type-7s to type-5s.

Hope that helps - pls rate the post if it does.

Paresh

[gopal_4476]

Hi Pkhatri

Thanks a lot. now i got the difference between nssa and normal areas in ospf domain.

Regards,

Gopal

[ankurbhasin]

Hi Paresh,

Just want to clarify something. As you stated in your statement "The type-5 will be generated by the NSSA ABR which could also be an ASBR"

But when I read this link it mention the condition for P bit to be 0 only when NSSA ASBR is also an NSSA ABR which means no translation from LSA7 to LSA 5.

"If bit P = 0, then the NSSA ABR must not translate this LSA into Type 5. This happens when NSSA ASBR is also an NSSA ABR."

Please confirm if I misunderatood your explaination?

In your statement which is in brackets you have mentioned ASBR is that ASBR the same which is generating Type 7 LSA or just another ASBR other then the one which is generating type 7 LSA.

Regards,

Ankur

[pkhatri]

Hi Ankur,

What I meant was that the ABR could also be an ASBR, but it cannot be the ASBR that injected that type-7 because in that case, the P-bit is zero.

Paresh

[ankurbhasin]

Hi Paresh,

Beautifull that was my understanding. I only got confused when you said about ASBR in brackets in your previous post and that is the reason in my last post I confirmed whether you are talking of ASBR who generates type 7 LSA or any other ASBR

Thanks for clarification.

Bingo 5!!

Ankur

[ankurbhasin]

Hey Paresh,

I don't know whats the problem I rated you 5 but it is not displaying and coming back to same result again and again.

Not sure whats happeneing out here!!

Ankur

[pkhatri]

Yeah, the server does that sometimes.

Sorry to have confused you earlier.. as you saw, I was just trying to state that the ABR can be a combined ABR/ASBR. This can be a pretty confusing setup actually. When you redistribute routes on an ABR/ASBR, it will generate both type-5 and type-7 LSAs and inject them into the appropriate areas.... I've had all sorts of fun trying out some of these weird scenarios !

Paresh