WAN Load Sharing with OSPF

[Kelvin Willacey]

hi,

I have a branch location with two links and I need one link to be primary for some routes and the other link to be primamry for others. Does anyone know of a way to manipulate specific routes using OSPF, much like what an offset list would do for EIGRP and RIP? I am trying to run away from using route maps or using separate OSPF processes in some way to accomplish this.

Ant thoughts? Thanks.

[cadet alain]

Hi,

As OSPF can't do unequal cost load balancing, the simplest thing is to higher the cost on backup link so it will only be used if primary fails.

But don't forget to configure the other routers if you can,  to alter return traffic.

Regards.

Alain.

[Richard Burts · Accepted solution]

The suggestion from Alain of setting the interface cost higher on one interface will certainly result in a primary and a backup interface. But the requirement in the original post was that one link be primary for some routes and the other link be primary for others. Changing interface cost does not accomplish this.

The original post is not clear whether the same set of routes are learned on both interfaces. Assuming that this the case then there is not a simple way in OSPF to manipulate the metric such as using offset list. As a link state protocol OSPF needs the metric to accurately reflect the topology of the network. We do not know much about the environment in the original post but I believe that some complex solution such as route maps or multiple processes will be required.

HTH

Rick

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[Kelvin Willacey]

Hi guys, thanks for the input. Richard you understand exactly what I am trying to accomplish. I figured my options would be limited, I wish OSPF was a little more flexible. I guess I will go with the multiple processes because I am not a fan of route maps, thanks. It would seem simple failover is no longer good enough these days.

[Richard Burts]

I am glad that my understanding of your situation was correct and that my response was helpful. OSPF is an effective routing protocol and produces good results with equal cost load sharing. But it does not help when you want to implement policy on the routing (want to prefer link 1 for routes 1, 2, and 3 while preferring link 2 for routes 4, 5, and 6). For arranging that kind of policy you do need things like multiple processes or route maps.

Thanks for marking the question as answered (and thanks for the points). It makes the forum more useful when people can read a question and can know from the rating that an answer provided a resolution. You have contributed to this process.

HTH

Rick