OSPF Area 0 Question

keeleym · ‎05-01-2009

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,

Michael

lamav · ‎05-01-2009

You're right, it's a bad design. If the PPP link goes down, you're going to have a split Area 0.

Victor

Harold Ritter · ‎05-01-2009

Victor,

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.

Regards

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

lamav · ‎05-01-2009

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.

Victor

Harold Ritter · ‎05-01-2009

Michael,

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.

http://tools.ietf.org/html/rfc5185

Regards

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Joseph W. Doherty · ‎05-01-2009

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.

[edit]

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

lamav · ‎05-01-2009

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.

HTH

Victor

keeleym · ‎05-06-2009

Hi All

Thank you all so much for taking the time to look at this and for the advice you all offered. It is much appreciated.

Best Regards,

Michael