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New Member

NSSA intra-area routing

I want to design an NSSA area with multiple ASBRS. My concern is multiple equal cost paths between the external networks. I do not want equal cost paths in the network. So I have a couple related questions...

1. In a TOTALLY stubby NSSA area, do the ASBRs within the NSSA area generate type-4 LSA's?

If not, how does intra-area external-to-external (RIP to RIP) routing take place, especially if there are parallel paths within the totally stubby NSSA area? With no type-4 LSAs and if parallel paths exist between external networks this would cause multiple equal cost paths if using default external type-2 LSAs. Would redistributing RIP routes to OSPF as metric type 1 (verses the default metric type 2) prevent multiple equal cost paths in this case? (assuming, of course, unequal cost paths)

2. In a "normal" NSSA area (NOT totally stubby) do the internal ASBRs generate type-4 LSAs?

5 REPLIES
Silver

Re: NSSA intra-area routing

There are no type 4's in NSSA or Totally stubby areas. Here are characteristics they have in common:

No Type 5 LSAs are allowed in the area.

All IGRP routes are redistributed as type 7. This type 7 can only exist within NSSA.

All type 7 LSAs are translated into type 5 LSAs by the NSSA ABR and are leaked into the OSPF domain as type 5 LSAs

Here are characteristics of the Totally stubby area (in addition to that above):

No type 3 or 4 summary LSAs are allowed in the area. This means no inter-area routes are allowed in the area.

A default route is injected into the NSSA totally stub area as a type 3 summary LSA.

Here is a very good link that shows information that should answer your inter-area routing question.

http://www.cisco.com/en/US/partner/tech/tk648/tk365/technologies_tech_note09186a0080094a88.shtml

Hope this helps you,

Don

New Member

Re: NSSA intra-area routing

Thanks for the reply! But it doesn't really answer all my questions.

If what you say is true.. (there are no type-4 LSAs in ANY kind of stub area, be it NSSA and/or totally stubby) HOW DOES 'intra-area' external routing occur in a "normal" NSSA and also a totally stubby NSSA?

HOW CAN I prevent equal cost paths between external networks inside a NSSA? WILL redistributing external networks via external type-1 LSAs help prevent this?

WOULD a 'normal' (NOT TSA) NSSA ABRS generate type-4 LSAs?

I have read and re-read technical papers, RFCs, books etc. and I can not see where it says ASBRs within an NSSA do not generate type-4 LSAs. ESPECIALLY in a "normal" NSSA. I HAVE SEEN that they are not allowed INTO this type of area from the backbone.

BTW I can't get to the link you suggested I should read. :-(

Silver

Re: NSSA intra-area routing

I just tried the link and it works fine for me.. might be an insider thing.. Please try this one again for it is good.

http://www.cisco.com/en/US/partner/tech/tk648/tk365/technologies_tech_note09186a0080094a88.shtml

To try to answer your questions a bit more.

No external (non default) summary LSA's are sent into a TS area. Just a 0/0 from the ABR.

Type 7s are from NSSA ASBR that transit the NSSA. NSSA ABR then translates this to type 5.

This links shows the Type 5 to 7 conversion (hope you can get to this one)

http://www.cisco.com/warp/customer/104/ospfdb8.html

Type 4 LSA (ASBR Summary LSA) are only allowed in normal Areas (covered in RFC 1587). Special type of LSA type 3, 3Default. It is to cover the information we lost due to making an area stub (i.e no type 4 or 5 LSA). Its is a default 0.0.0.0 route to the ABR and is injected in to the routing table automatically, when an area becomes a stub,tstub or tnssa.

Regards,

Don

New Member

Re: NSSA intra-area routing

I understand what you have said completely and thank you very much for your effort in your answers.

I DO understand how a NSSA area works and other stub areas as far as INTER-AREA routing goes and understand about type-7 LSAs etc. etc. believe me, I am well versed in OSPF operation. (at least 99%) That remaining 1% is what I am trying to learn about. LOL

My question is concerning INTRA-AREA routing within a NSSA.

For example, I have 3 ASBRs in a NSSA, lets say there are fully meshed and each one is injecting external type-2 routes into the NSSA. And to make it crystal clear, lets say all the links between these ASBRs do not have any equal cost paths between them. Due to there being no type-4 LSAs allowed to be generated by these ASBRs in ANY type of stub area, this would cause the routes between external networks within this NSSA to have 3 equal cost paths. Correct?

WILL redistributing external routes as external type-1 LSA prevent these equal cost paths?

From what I can understand from your answer all external-to-external intra-area routing inside a stub area would use the ABR as part of the path. Is this what you are saying. Excuse me for being a little thick headed :-) But from your answer, it suggests that other routers within the NSSA have no idea of any external routes being injected from other ASBRs in ther NSSA. I do not think this is true. Sorry if I misunderstood what you are trying to say.

THIS IS MY PROBLEM... I have not been able to find any information about INTRA-AREA routing inside a NSSA. :-(

Thanks again,

Jim Coffey

jmcoffey@charter.net

Silver

Re: NSSA intra-area routing

In RFC 1587 (section 3.5, paragraph 5) http://rfc.sunsite.dk/rfc/rfc1587.html

it states the following:

Otherwise, compare the cost of this new AS external path

to the ones present in the table. Note that type-5 and

type-7 routes are directly comparable. Type-1 external

paths are always shorter than Type-2 external paths.

Type-1 external paths are compared by looking at the sum

of the distance to the forwarding address/ASBR and the

advertised Type-1 paths (X+Y). Type-2 external paths are

compared by looking at the advertised Type-2 metrics,

and then if necessary, the distance to the forwarding

address/ASBR.

So type 1 would allow the different cost links to alter the metric of the route and should ensure External routes across non equal cost links are NOT seen as equal.

NSSA's would know of external routes from ASBRs within their area via the Type 7's. So from External to External path the ABR for the NSSA would not need to be in the path.

Hope this clears up your question on intra-area routing,

Don

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