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?
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. :-(
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.
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. :-(
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