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New Member

OSPF NSSA area, precision needed....

Hi !

for BSCI certification Exam...

When we are using the command :

area area-id  nssa command, we have the option to add parameter 'no-redistribution'.  I had understand this parameter disable the redistribution of route, but I don't know which one exactly : External route into NSSA area ? what will be interest ? when NSSA will have to speak with that network it will go back to area 0 (because the default route advertized by ABR) and after that, where they should go ? or any other area when they arrive on the router of the NSSA area router inside of that area they do not have the route to reach the external network where it is the only physical possible way to reach those network...

Also the author saying the ABR of an NSSA generate a default route but not by default, if it do not advertized that route how the NSSA can reach other area in the same AS if all external LSA are'nt advertized in the NSSA ? why the generation of the default route is not the default behavior ?

thanks a lot !

2 REPLIES
New Member

Re: OSPF NSSA area, precision needed....

I think that this document explains the redistribution issue quite well: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a88.shtml#filternssa. Please ask again if it is not clear.

The default route issue is because of the situation that you have an ABR and an ASBR in the NSSA. It is not clear on which of the two the default route should be generated. So therefore you have to decide where you put it with the default-information originate command.

HTH

mat

Blue

Re: OSPF NSSA area, precision needed....

Hi:

The purpose of the NSSA no-redistribution feature is to prevent an ABR that is also acting as an ASBR from propagating a type 7 LSA into the NSSA area. It's a type of route filtering. Parenthetically, it can be considered route filtering and not simply LSA filtering because it is being done at the ABR, which means that all the routers in the NSSA area will have the same LSDB.

When a router acting as both an ABR and an ASBR redistributes a route into OSPF, a type 7 and a type 5 external LSA will be generated. The type 5 LSA will not be propagated into the NSSA area, but the type 7 will. If you configure the no-redistribution keyword, the type 7 LSA will also not be propagated into the NSSA.

Also, the reason that an NSSA does not automatically generate a default route is that type 3 and type 4 LSAs are allowed into the NSSA, unlike a totally stubby area. So, inter-area summary addresses will be propagated across the ABR boundary into the NSSA. If you make the NSSA a totally stubby NSSA by adding the no-summary keyword, the type 3 and 4 LSAs will be blocked and no inter-area routes will be found in the route table. Therefore, the only way for traffic from an NSSA to reach the rest of the OSPF domain is through the default route that will be generated automatically once you configure the NSSA as a totally stubby NSSA.

In cases where you are allowing type 3 and 4 LSAs into the NSSA and need to reach external subnets outside the OSPF domain, you will need to configure a default route using the default-information-originate command at the ABR and inject it into the NSSA because type 5 LSAs from other areas in the OSPF domain will not be able to cross the NSSA ABR boundary.

HTH

Victor

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