EIGRP to OSPF migration

[Earl Granger IV]

Hi Community,

I was just trying understand the concept of migrating from EIGRP to OSPF.  In my scenario I am using BGP, OSPF and EIGRP in the core.  There is BGP for peering with Internet and MPLS obvisously and there is 2 OSPF areas.  One area redistributes from ospf to bgp and visa versa and the other area redistributes from EIGRP to ospf and visa versa.  In the redistribution process, there is some route tagging being done to filter some of the prefixes. Migrating these protocols by running both protocols at the same time and manipulating the AD is not that simple right?  Someone help with a little more insight.

Thanks

[JohnTylerPearce]

Earl,

So from my understanding, you have redistributing of BGP>OSPF and OSPF>BGP in your MPLS Core (whih sounds right), you have two OSPF areas, so I'm assuming one area is obviously ARea 0 and the other let's just say area 200.

So it sounds like this other area (for example area 200) is redistributing EIGRP into OSPF.

My first question is, is one router doing mutual authentication or are there two?

If it's just one router, then depending on how you have your route filtering configured, in theory, you could redistribute EIGRP into OSPF, then OSPF into BGP (MPLS Core), and then have it come back your way, depending on how you have things configured (route feedback). Since I don't know your network, that is something to look into and verify that the above cannot happen. (Just a friendly suggestion)

But if you're going to change the routers running EIGRP into OSPF, since EIGRP has a lower AD by default (90) for internal, External (180), you could configures OSPF on these routers, so they are running both protocols at the same time. The routes to EIGRP will inw out if they're are multiple (same route prefixes), since the AD of internal EIGRP routes is 90.

Are you redistributing OSPF into EIGRP as well?

[Earl Granger IV]

Thanks John,

I have two 6509s running HSRP and doing the redistribution, for some reason the redistribution configuration is not the same on both switches, especially if they are running HSRP.  Thats something that I would have to fix soon and investigate. 

Thats where the route tagging comes in to place, Its tagging routes and it filters them so they wont cause route feedback.

Yes, I see if I run both protocols at the same time OSPF will be less prefered in the beginning.  Can you re-clarify your last statement. "The routes to EIGRP will inw out if they're are multiple (same route prefixes), since the AD of internal EIGRP routes is 90."

Yes I am redistributing ospf into eigrp.

[Jon Marshall · Accepted solution]

I think "inw" is meant say "win" as in if you have the same route with EIGRP and OSPF the EIGRP route will be preferred.

However unfortunately it is not that simple. If you are redistributing OSPF into EIGRP then these routes will be EIGRP external ie. AD 170 so any OSPF route that is the same will be preferred.

It all depends on whether you are comparing OSPF to internal EIGRP in which case EIGRP is preferred or whether you are comparing OSPF to external EIGRP in which case OSPF is preferred.

Jon

[JohnTylerPearce · Accepted solution]

Earl,

I apologize, I probably should have worded that better.

Since, you are implementing route filtering, I'm assuming it's not going to allow the EIGRP routes advertised to OSPF, back into EIGRP. At least, I'm assuming that how you have it configured anyway.

If that's the case, then you could run OSPF on top of EIGRP, and the EIGRP routes will be preferred over the OSPF routes since they have an internal cost of 90 compard to OSPF's 110. Remember you will have two routes to (192.168.1.0/24), one OSPF and one EIGRP.

So you can safely run both at the same time, this will allow for an easy cut-over to OSPF.

Now if you go this route, then, you will still be redistributing OSPF into EIGRP, so you will have the other OSPF routes as External EIGRP routes with an Administrative Distance of 170. So you will have not only the internal EIGRP routes, and external EIGRP routes that are being currently redistributed, but also, the OSPF routes, since you will have OSPF runing as well. So in this case, the routes you are curretnyl redistributing into EIGRP from OSPF, will be less preferred than te OSPF routes due to same prefix and AD of 110.

I would lower the AD of all external EIGRP routes to below 110 so they will be preferred untill the cut-over.

You would need to do this in logical steps as well, to prevent any issues.

[Earl Granger IV]

Jon thanks,

I was thinking that as well.  Most of the routes are at the AD of 170.  I would have to manipulate the AD in a way right?

[Earl Granger IV]

Cool, I saw that late before I responded.

[JohnTylerPearce]

That would be correct EARL.

under the EIGRP process you can do the following.

router eigrp ASN

distance ?

And that will inform you how to change internal and external Administrative Distances.

[Earl Granger IV]

Thanks John and Jon!

[JohnTylerPearce]

Earl,

No problem, got luck with your future planning, configuration, and implementation

I would suggest doing this in the following steps

1. Modify the External AD of EIGRP routes on the EIGRP routes being migrated to OSPF, to a value less than 110.

2. Check on your route-feed and make sure that looks good

3. Configure the OSPF process on routers that are being migrated from EIGRP to OSPF.

4. Verify that all the other OSPF routes are in the topology database

And go from there

[Earl Granger IV]

Thanks again dude.