IP OSPF Priority

[Yannick Vranckx]

Hello,

As part of an upgrade it was needed to change the priorities in OSPF because we were not happy on the locations of the DR/BDR

I

So before the upgrade the situation was as follows:

Core 1 : DR

Router 2: BDR

After the upgrade i wanted the situation to be:

Core 1: DR

Core 2: BDR

But it turned out to be the following: Core 2: DR and Router 1 BDR

So both cores route OSPF with a specific vlan to the routers and the routers have sub interfaces, this is due to having multiple types of traffic.

So before this upgrade we wanted to fail over the OSPF traffic to use a specific way across Core 2 instead of Core 1 because that chassis would be rebooted

I changed the priority of Core 1 from 10 to 0

I change the priority of Router 2 from default to 0

I reloaded the chassis on Core 1.

I would expect that Core 2 would become the DR as it did with a priority if 9 and Router 1 the BDR as it had the default priority. When Core 1 came back i changed the priority back to 10 and back to default on Router 2. 

But Core 2 remains the DR and Router 1 is now the BDR, i know the DR/BDR is there to reduce the flooding of the LSA's and it has no impact on how we route traffic across the WAN. We do that with costs.

But for a cleaner topology i would wanted both cores to be DR/BDR, where did i make the error?

Do i have to put Core 2 in a prio of 0 before it fails over to Core 1? And then both routers for a fail-over to Core 2 when i restore the priority. I don't think OSPF to preempt.

[paul driver]

Hello
Core 1 needs a higher priority on the link to Core 2, and then both cores need higher priorities on the interfaces towards the rtrs, Possible make the rtrs drothers by setting their interfaces with a priority of 0

Clear Ip ospf process


res
Paul

[Yannick Vranckx]

Hi,

I think this was my mistake.

I left Router 1 to be default instead of going for 0. 

But in any case the OSPF Priority selection does not preempt right? So even if Core 1 has Prio 10 and Core 2 has Prio 9, i reload the core. It will not preempt the role DR away from Core 2?

[paul driver]

Hello

OSPF Priority selection does not preempt right?

That is correct no preemption - to change the role,  Manual restart of the interfaces or the ospf process is required

res
Paul

[Joseph W. Doherty]

Correct, OSPF priority does not preempt.

i know the DR/BDR is there to reduce the flooding of the LSA's and it has no impact on how we route traffic across the WAN. We do that with costs.

Also correct, but it's also to avoid the need for multiple routers to process a full mesh of LSA updates.  You also realize, the priority feature, for selecting DR/BDR, is really intended so we can help manage OSPF processing load, per device.  I.e. the DR "should" be the router with the most available CPU cycles.

In my experience, on modern equipment, the DR/BDR role doesn't seem to add much to a router's normal work load.  In pathological cases, it might be important, but such cases often should be precluded via other techniques rather than worrying about optimal DR/BDR assignments.