Migration from EIGRP to OSPF

[paulmoesher]

I am tasked with migrating a network from EIGRP to OSPF. My thought is that I would start configuring OSPF on the exterior and work my way to the core. That being said my end goal is that the core be the designated router. I am worried that if I start enabling OSPF on the exterior first then another router would become the designated router before I get to the core, even if I configure the core with a higher OSPF priority due to the fact that an OSPF DR has already been elected. I know that once I get to the core I can start shutting interfaces and restarting the OSPF process to force the core to be the DR but I really do not want to go that route.

My current idea is that I figure all exterior routers with an OSPF priority of 0 then once I get to the core configure that with an OSPF priority of 2 then the routes will be learned and the core will be the designated router.

My only worry is that if I set all of the exterior routers with a priority of 0 will they still form neighbor relationships with each other and then cause redundant traffic to be passed around the network until I configure the core due to the fact that there is no designated router?

Additionally, if I go this way then there will never be a backup designated router. How important is it to actually have one of your routers be elected as BDR in OSPF?

[Georg Pauwen]

Hello,

how many routers are we talking about in your topology ? You can set the OSPF priority to determine exactly which router becomes the DR, and which the BDR. Unless you are dealing with a massive amount of routers, it might be advisable to 'migrate' everything at aonce.

[paulmoesher]

There are about 30 routers in the topology.

[balaji.bandi]

how is your network, do you 2 Core Routers or only 1 ?

If i have 2 Core Router i go with p2p link so you can avoid this election process,

If you looking DR and BDR option then you can configure one of the Core as DR and BDR, Rest of the Device now elect as DR or BRD.

You can influence the DR and BDR election process by manually configuring the OSPF priority. This is done by using the ip ospf priority VALUE command interface command.

[paulmoesher]

I only have one core router in my topology.

[paul driver]

Hello
I have performed this on a couple of occasions with success.

You can achieve this by running eigrp and ospf side by side and what is required is to set your existing ospf administrative distances to be lower than eigrp process this way your existing routing should not be affected and at the same time you should be able to validate the ospf adjacency's and topology tables.

.

[MHM Cisco World]

I dont know but i have idea 

Start config from exetrnal with config auth for ospf but without password,

Until you finish config in core start from core add password until external.

[paul driver]

Hello

@MHM Cisco World  can you elaborate on this

If the OP has an existing igp and they want to migrate to alternative igp by applying authentication to an ospf process what will this achieve?

[MHM Cisco World]

...

[Richard Burts]

The original post exhibits a belief that is common when people are learning about OSPF that there would be a single  DR for the OSPF domain. As was pointed out in a previous response the OSPF DR/BDR are elected per multi access segment in the network. So most OSPF implementations will have multiple DR. A network segment at the edge would have its own DR/BDR, a network segment in the middle wold have its own DR/BDR, and the network segment at the core would have its own DR/BDR. So election of DR/BDR is not much of a concern in converting the network from EIGRP to OSPF.

One interesting aspect of converting a running network from EIGRP to OSPF is that as you implement OSPF it will run in parallel with EIGRP and the EIGRP routes will be preferred (based on the AD of the protocols). So implementing OSPF will not have much impact on the running network, and you will have opportunity to verify that OSPF is running as expected, that OSPF adjacencies are formed as expected, that entries are created in the OSPF data base as expected. After you have verified that OSPF is performing as expected then you can switch over from EIGRP to OSPF. You can do that by changing the AD of EIGRP to be lower than OSPF or by removing EIGRP from the router configurations.

You could certainly do the conversion from the edge to the core or from the core to the edge. My preference would be to do it from core to the edge.

[Jon Marshall]

Just wanted to add, EIGRP routes will be used unless they are EIGRP external and then OSPF would be used so there is a need to be careful and as Paul suggested change AD of EIGRP external. 

To the OP,  I would do as Paul suggested and run side by side, but you will need to modify EIGRP external to be < 110 for that to work. 

Jon

[paul driver]

Hello
I would say incorporating authentication into OSPF isn’t a necessity to migration but it can be part of it if desired However you would want to obtain adjacency's to verify the ospf table to see if it is getting populated with the correct prefixes expected before you flip the administrative distances so ospf is preferred and its prefixes are entered into the GRT

[Joseph W. Doherty · Accepted solution]

Just want to confirm, you realize a DR (and BDR) only need to come in to play when you have more than two OSPF routers on the same shared segment/network.  (If using Ethernet, and if you only have two routers with interfaces within the same shared segment/network, you can define the interfaces as OSPF p2p. [Avoiding DR/BDR altogether.  Also, often a "good thing" to avoid having more than two OSPF routers on a shared segment/network.])

[paulmoesher]

That makes sense to me. There are some router interfaces that share a network segment. I am not 100 percent sure what the logic is behind that as this network is new to me. However, for any router interfaces that only share a subnet with the directly connected interface on the core, I will configure it as p2p as you suggested.

[Joseph W. Doherty]

BTW, from the OSPF perspective, the connecting network doesn't need to be a /30 or /31, to declare the OSPF interface as p2p.