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OSPF & NSSA

I've configured OSPF on 3 switches - Core and 2 access switches. All switches are in area 1.

I wish to propagate default route 0.0.0.0 from my core switch to access switch. But without area 0 configured on the core, the default route 0.0.0.0 cannot be propagated to the access switch. Therefore, I configured my Core switch as a NSSA ASBR to propagate default route to its switches. Is that the correct way to do it?

4 REPLIES
Cisco Employee

Re: OSPF & NSSA

Hello Alan,

This probably works for you but it is not the correct design.

It is not true that the default route can be propagated only in area 0. A default route can be propagated in any area and in any area type.

The correct way to introduce default route into your OSPF domain is to use the OSPF command default-information originate on your router that connects your OSPF domain to the outside world (the ASBR). This command requires that the router already knows the default route, otherwise, the default route will not be injected into OSPF. If you want that router to send the default route no matter what, use the command default-information originate always.

You should remove the NSSA type from your area and leave the area 1 simply as a transit area.

Best regards,

Peter

New Member

Re: OSPF & NSSA

Hi Peter,

I have no area 0s, but only an area 1 configured. The problem is that I want the access switches exit from the core switch. The area 1 is supposed to be a totally stubby area from my design. It has evolved into something I felt that it was incorrect. But because if no area 0 is configured, "default-information always" won't propagate routes to those access switches right?

Cisco Employee

Re: OSPF & NSSA

Hello,

In stubby and totally stubby areas, it is the responsibility of the ABR to inject the default route (and it will do it automatically, without any additional configuration). You are correct, the "default-information originate" command will not work on a router that is internal within a stubby/totally stubby area.

Why do you require your area 1 to be a totally stubby area? Note that without area 0, making other areas to be of some special type does not make much sense. Do you need to maintain your current area assignments?

Best regards,

Peter

New Member

Re: OSPF & NSSA

Hi Peter,

You posted a very good question on why I need a totally stubby area when I do not have area 0. It makes sense to me now. Haha. Now I am doing without the totally stubby area and it works very well for me.

Thanks Peter. Appreciate it.

Regards,

Alan

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Why do you require your area 1 to be a totally stubby area? Note that without area 0, making other areas to be of some special type does not make much sense. Do you need to maintain your current area assignments? ##############################

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