Migrating from EIGRP to OSPF

[dtran]

Hello everyone, hope everyone is doing well !!!

Thanks in advance for inputs/suggestions !!!! I am looking into migrating from EIGRP to OSPF inside my data center and needed some guidance.

I have been running EIGRP inside my data center forever and BGP on the WAN side. For the past few years I have the needs to exchange routes with 3rd party devices inside my data center and I've been able to BGP peer with these devices and do route redistribution with EIGRP and all has been working fine.

I am thinking to simplify routing inside my data center since these 3rd party devices support OSFP that way I only have to deal with 1 routing protocol inside the data center and not have to deal with route redistribution. I'll just have to learn OSPF which I have not worked with in the past, but that's not the concern (just another routing protocol). Is this the right path to take ???

I appreciate any inputs / suggestions !!! Please share any OSPF sample configs or documents that you may have. Thank you so much !!

Danny

[dtran]

Hi MHM, 

I am running L3 access-aggr-Core. 

Thanks MHM !!! very much appreciate your help !!

Danny

[Richard Burts]

Danny asks " Would I be using the backbone area 0 in my scenario ?" As Joseph has indicated if you configure just a single area then it can have any area number. My suggestion would be to start with area 0. As the network grows and it is appropriate to utilize other areas you can assign router interfaces to the appropriate area.

[Joseph W. Doherty]

Hmm, Rick's recent reply made me "think" about how he is approaching a single area to multi-area conversion, vs. what I had in mind.  I suspect our approaches are different due to where we want to be after such an initial conversion.

For me, after such a conversion, area zero routers would only be those needed for area zero transit purposes.  For Rick, and others (?), I suspect they would, at least initially, leave all your (at time of conversion) area zero routers in area zero.  (NB: of course, many of the original one area, area zero, routers could be later migrated to a different area.)

What I had in mind, if all your routers were in a non-zero area now, and you needed to move to a multi-area topology, I would only add area zero to a couple of interfaces to your core devices, making them ABRs, and then add other areas to those routers, as needed.

What I suspect Rick has in mind, is by keeping all the existing routers in area zero, all you need to do, to add another OSPF area is connect such a new area's ABR to ANY existing area zero router (including, if desired, your core devices), or, changing an interface on ANY existing area zero router (including, if desired, your core devices) making it an ABR for the newly added area.

Any/all of the above will work, with minimum conversion required.

However, personally I like to keep OSPF area zero restricted to only using its routers for area zero transit purposes.  I.e. I would not want to see all your "other" data center routers, in area zero, once you move to a multi-area topology.  (Somewhat similar thinking to not having hosts directed connected to core devices either.)  So, to avoid extra migration effort (i.e. moving a bunch of original area zero routers to a non-zero area), that's why I suggested not using area zero until you migrate to a multi-area topology.

Again, though, you're network is so small, this really shouldn't really matter, and whatever you feel most comfortable with, for how you want to do this, should work fine.  I'll also add, I believe, most network engineers would go with using area zero now (probably due to thinking you design from the core, or OSPF area zero, outward).

[Richard Burts]

There are multiple approaches that could be used for the design of the new OSPF network. We do not know much detail about the network which makes it difficult to determine if one approach is more appealing than another. There were several factors that lead me to suggest starting with area zero. The first factor was what Danny told us in the original post "I am looking into migrating from EIGRP to OSPF inside my data center". It seems likely to me that the network devices in the data center would be the core devices that eventually would be placed in the area zero. So why not start with them in area zero? Another factor was thinking that if you start with some non zero area and then later want to more the core devices into the new area zero that it might be more impact on the network. Whereas if you start with the core devices in area zero it is less impact to more some devices out of area zero and into new areas. Another factor was the thought that perhaps as the network grows and Danny wants to transition from a single area to multi area he might want some of the new areas to be some type of stub. I believe that it would be easier to create new areas as stub and move devices into them from the area zero and more complex to start with a non zero area which later might become a stub.

Certainly both approaches are viable. I have an opinion, Joseph has an opinion. I do not think that there is any right or wrong in either opinion. Danny can consider the alternatives and choose what he prefers.

[Joseph W. Doherty]

Ricks puts it very well.

We're both "guessing" how your network might evolve (assuming it does), and based on our "guesses" we're both trying to avoid as much migration impact moving to a multi-area OSPF topology.  Worse case, if you go contrary to either of our "guesses", and the approach supporting them, it only means just some more work to migrate.  I.e. no deal stoppers.

For right now, again, a single area's behavior doesn't matter whether it's area zero or not.  I.e. so for right now, no difference whether you use area zero or not for your single area.

I will add, if you do move to a multi-area OSPF topology, there can quite a jump in possible complexity (like different area types, like Rick mentioning ". . . new areas to be some type of stub."), and OSPF operational issues because of such, much of which is not obvious at first glance.  If you do get to that point, it might be advisable to consult with someone local, who can come on-site to assist you in what your multi-area OSPF topology should be.

I'll also add (again?), Cisco's OSPF single area topologies can usually scale much larger than the (very) old rule-of-thumb, of no more than 50 routers per area (and that's not even accounting for Cisco's recent OSPF iSFP feature or tuning their OSPF stability parameters).

[MHM Cisco World]

https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Campus/routed-ex.html
this doc. help you in your case 

[Joseph W. Doherty]

Good reference.

BTW, if you look at that reference's OSPF Area Design you might note how the data center (area 40) is not included in area zero, or the campus backbone.  This might appear to make what Rick suggested, keeping your DC in area zero, somehow "wrong", but he is not.  Again, you can migrate DC routers into their own area (yea, perhaps some extra work), but I suspect Rick might also have in mind that your overall network being so small, having the DC within area zero, isn't really bad because logically, the DC is the core of all your services.

So as Rick also noted in his recent reply, there's much detail we don't know, making it difficult to chose an approach, and even if we did have much more detailed knowledge, often much of network design is, as Rick also notes, a matter of opinion, often without really one being "right" vs. the other being "wrong".

[MHM Cisco World]

@Joseph W. Doherty  sorry but may be you miss reading my previous comment 
"""Use Area 0, In feature when you expand your network you dont care about the split area and split backbone area, meaning no need for virtual link.
where Backbone area 0 , if I need multi area ?
if you run 
Access-Aggr-Core 
between the Access and Aggr run any area other than area 0 
in Aggr-Core run area 0"""

here area 0 contain all Core and AGGR router for all 
Data Center, WAN ...... etc. 

and area 40 in doc. is for AGGR-ACCESS not AGGR-CORE. 

[Joseph W. Doherty]

I did see your earlier posting.

Firstly, what I thought my reference linked to was inside of your document reference, which should be (?)

In the above, "Data Center" appears to be in "Area 40".

The point of my prior posting was, this diagram, having DC in its own area, seems contrary to what Rick was recommending, i.e. using area zero for a one area topology, at at least initially, keeping DC routers in it.  Although, in fact, this diagram is contrary, I tried to say, what Rick recommended wasn't "wrong", like he notes, there's much we don't know about OP's network, and future plans, and there are possible reasons why to keep DC routers in area zero contrary to this diagram.

My posting, didn't intend to be relevant to any of yours, and it it appears contrary to such, I didn't realize it was.  That said, briefly scanning that whole document, I couldn't find "area 40 in doc. is for AGGR-ACCESS not AGGR-CORE. ", where's that at?