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The purpose of OSPF  NSSA and NSSA totally stub

Posted on 2014-09-19
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In OSPF there are many area types:

Stub area : will block external routes LSA type 5, but allow inter area routes LSA type 3 and 4 and injects default route

Totally Stub area: Block External as well as Inter area routes, and inject default route.

Well if I understand the purpose of creating the above areas is kind of reducing the size of the routing table. if that's the case why not just use the Totally Stub area and that will do the job ? I mean without worrying about creating the stub area .

OSPF also has NSSA and NSSA totally stub, when do we need these areas ? I know that NSSA when receiving the redistributed external routes it will mark them as Type 7, then the ABR will convert them to type 5 LSA, but what purpose it serves if it does not reduce the size of the routing table ? Type 7 or Type5 will not reduce the size if I understand..

Thank you
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Question by:jskfan
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by:mikebernhardt
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In answer to your first question, yes you could use totally stubby everywhere. You just get less granularity, where the default route might not be the optimum path to some other OSPF area. Totally stubby is a Cisco-proprietary extension (I think) which goes a step further than the RFC.

NSSA allows an area to redistribute routes directly into the area as you said, while still acting as a normal Stub area when it comes to OSPF. NSSA totally stubby serves the same function as totally Stubby, but also allows the routes redistributed into it.

Also note that in OSPF, it's really the LSA database that gets reduced, and the routing table follows.
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by:jskfan
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instead of NSSA area we can just have a normal area... followed by Stub area or better than that by Totally stub area if we want the database or routing table to be smaller
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by:mikebernhardt
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NSSA is only useful if you expect to be redistributing routes into a stub area. And whether or not you even bother with stub areas really depends how big your network is. Unless it's got hundreds of routes, there is not really a good reason to bother other than the automatic injection of a default route. But somewhere you probably already have a default route and that's where it would be injected with "default-information originate."
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by:jskfan
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This is part of Learning only....
I just wanted to know when should we resort to NSSA or NSSA Totally Stubby.
Knowing that the normal Totally Stubby area can inject the default route and block LSA 3,4,5,7

Regarding "default-information originate", it does not filter out any LSA

if you create it in Router1, I believe other routers ,(I am not sure if they have to be in the same segment and/or same area ) will have a default route created in their routing table.
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default-information originate has the sole purpose of injecting a default route into the LSA database and it's distributed throughout. It doesn't filter anything, as you said. It only works if you put it on a router that has an actual route to the internet via static route or BGP. Or you can use "default-information originate always" which will inject it even if there is NO path (not a very good idea in most cases).
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Thanks
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