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OSPF Area 0 Question

Unanswered Question

Hi All

Can you please see the attached diagram and tell me if I am correct in my assumption that I am dealing with discontigious Area 0 areas and a virtual link will be required, or is this design totally flawed?

This is a design that I was asked to look over and I do not think it is a very good design.

I know that there is a mis-match between the IP subnets on either side of the PPP link between routers. But this is as per the diagram I was given.

Best regards & TIA,


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Overall Rating: 4.6 (5 ratings)
lamav Fri, 05/01/2009 - 05:27
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You're right, it's a bad design. If the PPP link goes down, you're going to have a split Area 0.


Harold Ritter Fri, 05/01/2009 - 05:41
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Area 0 is split even if the ppp link is up as this link is in area 3. Updates coming from area 0 on one ABR will be converted to inter area routes and will be propagated via area 3 but will not be used by the other ABR, as stated by RFC2328. The ppp link could be moved to area 0 to fix the problem.


lamav Fri, 05/01/2009 - 09:38
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Hi, Harold:

Thanks for the clarification. I actually just realized that you were addressing me in one of your posts after I submitted my correction below. Thanks for the heads-up. I read his diagram wrong.


Harold Ritter Fri, 05/01/2009 - 05:36
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Your assumption is correct. Area 0 is indeed discontiguous. You could configure a VL between these two routers. Another way to address the issue would be to run back to back FR on the link between the two routers, instead of the ppp encapsulation. This would allow you to configure multiple subinterfaces that you could then attach to area 0 and area 3.

Also, RFC5185 allows you to configure any given link in multiple area. This RFC is already implemented in IOS-XR.



Joseph W. Doherty Fri, 05/01/2009 - 08:50
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As the other posters have noted, your area zero is split. Besides using a VL, it's unclear why the PPP link is in area 3. From the diagram, alone, another option would be to place the PPP link into area 0 instead of area 3.


Oops, didn't, until now, noticed Harold already suggested moving PPP link to area 0.

lamav Fri, 05/01/2009 - 09:36
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A mistake on my part...thought the PPP link was in Area 0. Its still a bad design. Actually, its worse, since you dont even have one link connecting the two Area 0s. So, you are already discontiguous and would need a VL.

As a rule of thumb, any design that relies on a VL to provide Area 0 contigutity is a bad design.

If your Area 0 routers are geographically separated, (ie you need a WAN link), you should have a high availability design with multiple WAN connections, with as much carrier and space diversity as possible.




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