OSPF NSSA question

Unanswered Question
Oct 10th, 2009

Suppose I have the following scenario:

Rip distributed into OSPF area 2,

which connects to OSPF area 0

OSPF area 3 an NSSA area also connects to area 0

Would the distributed RIP routes from area 2 be seen in area 1?

Since the RIP routes would be seen as E2 routes in Area 0, would they be seen as lsa 7 routes in area 2?

Also, in which case would an NSSA area translate the lsa 7 to an lsa 5?

I have this problem too.
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Giuseppe Larosa Sat, 10/10/2009 - 08:20

Hello Richard,

NSSA Area are used in the opposite way:

they don't accept external routes coming from area 0.

But NSSA ABRs can convert O N1 or O N2 LSA type 7 to LSA type 5 to be injected into area 0 and to be propagated to all non stub areas.

So you can redistribute inside an NSSA area but external routes coming from area0 are blocked at area boundary.

Without NSSA redistribution would be allowed only in area0 and in standard areas.

Hope to help


wilson_1234_2 Sat, 10/10/2009 - 09:59

yes thank you Giuseppe,

I think I see,

distributed routes can exist in the NSSA only when they are directly injected from ASBR, correct?

another question:

When the external routes are converted to a "default route" from area 0 to NSSA, is the default route, or is it a summary route of the distributed subnet?

Richard Burts Sat, 10/10/2009 - 10:10


Yes distributed routes can exist in the NSSA only when they are directly injected from ASBR (which exists within the NSSA area).

Perhaps there is some aspect of the rest of your question that I do not quite grasp. But in my experience a "default" route is always a route.

Who would be creating the summary route of the distributed subnet? If the distribution is in area 0 then the summary must be in area 0. And neither the distributed subnet nor its summary would be advertised from area 0 into an NSSA area.

If you are asking about some aspect that I have not grasped then perhaps you can clarify the question.



Giuseppe Larosa Sat, 10/10/2009 - 10:35

Hello Richard,

being the NSSA a stub area if the ABR router is configured to inject a default-route into the NSSA area:

router inside the NSSA sees a route originated by ABR node of type O IA or O N2 (not sure about this it shoulddepend from some factors: O N2 if using default-originate option, O IA if using no summary option).

The key point is that the real OSPF default route (O E2 or O E1) used in the OSPF domain cannot be passed in the NSSA area there is no exception to the block of O E1 and O E2 routes at area boundary, not even for the default route.

Hope to help



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