How to configure OSPF Totally Stub Area

Hi Rene,

As I read some article about STUB, Stub-Summary, NSSA, NSSA-Summary. It just block External Route or Internal Route or Both to advertise, but it always inject default route to destination. the purpose the new these feature to reduce amount of cpu hit. Am i right ?

Please explain more about it.

Hi Chheang,

Not all “stub” types will insert a default route.

Areas that only have one exit point (area 0) don’t need to know all internal routes. By using the stub areas, we can reduce the size of their routing tables. A smaller routing table means a reduced memory requirement.

Rene

Hi Rene

For the Totally Stub area, the default route is a Type 3 LSA. But Type 3 LSA is not allowed in Totally Stubby area.
This seems like conflict, I was just wondering. If I am wrong about this, please do let me know.

Thanks for the great work.

Palani

Hi Palani,

It does sound conflicting yes but that’s how it works. The default route is advertised as a LSA type 3 (summary LSA) in the totally stub area. All other summary LSAs are not permitted though.

Rene

Why exactly is the ABR the only one in a stub area to require the “no summary” keyword?
I lab’d it up in GNS3 and your right the others dont need it to work and be a stub area, I was just curious as to why?
Why dont the others in the stub area require “no summary”?
Thanks

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Rafael,
The difference between a Stub and a Totally Stubby Area occurs only at the ABR (the ABR determines what type of LSAs to allow into the Stub / Totally Stubby area). Since from the perspective of routers within the area there is no functional difference, there was no need to have extra information which might otherwise break the routers from forming a neighbor relationship.

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Rene,

Got my response to the question which I posted in the stub area page.

Love your work, as always…

I have learned most of my topics by watching your gns3vault & now network lesson is to the next level.

Kudos to you for all your efforts!

Hi Udaya,

Happy to know that Rene’s lessons are helping you with your study. Your message will be forwarded to Rene.

Thanks.

Hi Rene,

Nice explanation, The Last output:

R2#show ip route ospf 
O*IA 0.0.0.0/0 [110/2] via 192.168.23.2, 00:02:27, FastEthernet0/0

Is it supposed to be output of R3. Kindly check and update. Thanks.Preformatted text

Thanks, just fixed it.

In a Totally Stubby and Totally NSSA area, only the default route will be advertise as a LSA Type 3 by the ABR? No other LSA Type 3 is advertise, correct?

Hi Keng,

That’s right, all inter-area routers (LSA type 3) are blocked.

Rene

Hello ,
I have a question. When you enter ‘area 1 stub no-summary’ command what happens at the backend so that router does not forward summary LSA into the area. I mean to say how would router come to know not to forward type 3 (Summary) LSA. For example: In case of Stub area there is E bit set in Hello packet to make it stub and block type 4 and type 5 LSA. What is different for totally stub area? any additional bit or flag?

Hi Ravi,

The area 1 stub no-summary command is only configured on the ABR, on all other routers in the totally stub area you can use the normal area 1 stub command.

The only router that needs to know that we don’t forward LSA type 3 in the area is the ABR, and that’s where we configure the command. Once you enable it, it doesn’t forward LSA type 3 and other routers will never know it.

Rene

@ReneMolenaar/@lagapides

At the end of the this topic, you wrote “You only have to configure this on the ABR, we don’t have to make any changes to R2.” I think it should be R3 instead? :slight_smile:

Hello sales2161

Yes you are correct, thanks for catching that. I’ll let Rene know to make the change…

Laz

@sales2161 You are correct, just fixed this. Thanks for letting us know!

Rene

After reading the tutorial, I’m a bit confused about the configuration for Totally Stub Area and Totally NSSA.
How do we determine which router to run “area X stub no-summary” or “area X nssa no-summary”
I can see that no-summary has been run on the middle of router but what if we have many routers.

Hello Po

First of all, all four stub area types (Stub, Totally stub, NSSA, and totally NSSA) are characterized by the fact that they only have a single ABR. Remember, an ABR is an OSPF router that has two or more areas connected to it. This essentially means that there is only one entry and exit point from that particular area. This also means that area 0 can never be a stub network, because it is the backbone network to which all stubs connect. Remember that all ABRs must have at least one interface in area 0 for OSPF to function correctly. For a refresher, take a look at the following lesson:

Since all stub areas have only a single ABR, this means that these commands must be implemented on the ABRs that connect to the stub network in question, and must be applied to the area connected to that ABR which is a stub.

Looking at this topology from the lesson:


If we want to configure area 1 as a totally stub area, the command must be implemented in the ABR connected to that totally stub area. R2 is the ABR connected to that totally stub area, and by definition there is only one ABR in area 1 (otherwise it would not be a stub). Secondly, the command must be implemented on Area 1. So in R2, you would implement:

area 1 stub no-summary

Note that this command must be implemented only on the ABR. R3 is not an ABR, but must be configured as part of a normal stub area, this is why the command area 1 stub is configured there.

I hope this has been helpful!

Laz

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Thank you very much for your detailed explanation!

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