How to configure OSPF Totally NSSA Area

This topic is to discuss the following lesson:

Hi Rene !

When you configured the area 1 nssa no-summary you have not configured the same statement in router Susan. Why not ? Its not also necessary? or i am misunderstanding and you have to configured in the ASBR only ?

You said that we have to configured the area 1 nssa-summary in all the area. So i have that doubt.

THANKS IN ADVANCE !!

Hi Ieltxu,

Good question. You have to configure “no-summary” only on the ABR since that’s the router that will inject the default route(s) into the area. No need to do it on all routers in the (totally) NSSA area.

Rene

Rene lets say i have 3 routers on Area 1, to have a totally NSSA, i only need to configure area 1 stub nssa on 3 routers? then the abr is area 1 stub nssa no-summary?
so in short, the ASBR must be configured “area 1 nssa” while the ABR is “area 1 nssa no-summary” to achieve the Totally NSSA?

Hi John,

That’s right, within the NSSA area all routers only need “area x nssa”. Only the ABR requires “area x nssa no-summary”.

Rene

Hi Ajith,

That’s right, it’s the ABR that will block the LSAs from entering the stub or nssa area so that’s the only router where you have to add “no-summary”.

Rene

Hi Rene,

Could you please cover more on forwarding address?

Hello Mobasshir

The forward or forwarding address is an address that a router may advertise to another router as an alternate next hop. In most cases, advertising routers will state that they are the next hop, so the forwarding address field is left null (0.0.0.0) suggesting that the route is reachable only via the advertising router. This is useful in some cases especially when an External LSA by an ASBR is used.

More about the forwarding address and how it is determined can be found as part of this lesson:


Cisco also has a comprehensive description of the forwarding address and the methods used in its selection in the following documentation:

I hope this has been helpful!

Laz

Hey Rene,

First off, I just wanted to say that I really love the pin feature that pins the diagrams to the lower right side of the monitor. It has made studying so much more convenient on your site!

Anyway, I just wanted to point something out on this lesson. In the passage below:

“When we look at R3 you’ll see network 192.168.12.0 /24 as inter-area (LSA Type 3) and 1.1.1.0 /24 as external type 2 (LSA Type 5). First we’ll create a loopback interface on R3 and redistribute it into OSPF to show you the NSSA area.”

I think it should read:

“When we look at R3 you’ll see network 192.168.12.0 /24 and a default route as inter-area (LSA Type 3)”.

Since area 1 has been configured as a stub area, we won’t see 1.1.1.0 /24 as external type 2 (LSA Type 5).

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Hello Leroy

Glad to hear that you like the pin feature. We’re always trying to make the site as functional and useful as possible for you to study and get the most out of the content. If you have any suggestions to further improve its functionality, let us know!

Thanks for pointing this typo out, I will let @ReneMolenaar know to make the correction.

Thanks again!

Laz

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