OSPF DR

[bgrove2913]

I have 3 routers, A, B and C. If I want to make sure A is DR in any case, can some configuration be done only on A without configuring "priority 0" on B and C?

Thanks

[Harold Ritter]

This would certainly make sure that router A becomes the DR. The problem is that if router A goes down, none of the two other routers will take over the DR functionality and the transit network will become inoperational.

Hope this helps,

[Richard Burts · Accepted solution]

Harold has answered part of your question. I would like to answer the other part. Harold has pointed out a disadvantage of configuring priority 0 on B and C. Your question was whether there was anything to configure on A that would assure that it would become DR and not require configuration on B and C. I believe that the answer is NO there is not anything to configure on A that will assure that it will become DR. Unless you prevent B and C from becoming DR (by assigning priority 0 to them) there is a scenario where B or C becomes active on the segment while A is not active. In that case B or C would become Dr and there is nothing that you can configure on A that will change that or that will allow A to authomatically become DR when it does become active on the segment.

You could achieve your desired results through manual procedures but not through some configuration option.

HTH

Rick

[jrahm]

Instead of setting routers B and C to priority 0, eliminating them from consideration, why not set router A to priority 2, ensuring that it is always elected over routers B and C, which would be the default priority 1?

[Harold Ritter]

As Richard stated in his posting, setting router A's priority will help having this router elected as the DR but doesn't guarantee it.

If the DR election has already taken place between router B and C when router A comes online, the current DR will remain in function. This behaviour has been chosen for stability purposes.

Unlike OSPF, ISIS will always trigger a DIS (DR equivalent for ISIS) election once a new router comes online on a broadcast network.

Hope this helps,

[jrahm]

Gotcha, thanks.

On a side note, if router B and C should never be elected on that subnet, why run OSPF on those interfaces?

[Harold Ritter]

That is a good point. I had already commented on what would happen if router A was to go away and none of the other two routers was being elected as the DR. This would have the effect that this transit network would become inoperational.

Hope this helps,

[Richard Burts · Accepted solution]

I believe that there are situations where you do want to run a routing protocol like OSPF on the routers and do want to prevent B or C from being elected DR. Take for example some NBMA network where A is the hub, B and C are remotes that have subnets to advertise (and perhaps even have routers downstream from B and C that need to participate in OSPF). In this situation you do want to run OSPF over the links, do want to allow only A to be elected DR. In this situation if A is functioning the network works and if A is not functioning then B and C (and their downstreams) are isolated from the rest of the network.

HTH

Rick