Converting from Eigrp to OSPF

[karl.jones]

Hi,

I am currently running a contiguous EIGRP network. There are 3 routers in the backbone, routers A,B, and C all directly connected via a lan connection. Various remote sites connect to any of these routers via different media types such as isdn, frame etc. The network between A & B and A to remote sites is 172.16.x.x. The connection between B & C and C and remote sites is 172.22.x.x. This makes the network contiguous. The problem i have is that if i want to take a frame connection from a remote 172.16 site to router C for example, the network becomes discontiguous, hence i want to convert to OSPF so i can have 3 routers in my backbone and have any remote sites (either 172.22 or 172.16) connecting to the backbone routers of A, B & C, as long as i have configured the two networks (172.22 & 172.16) in different areas, for instance area 1 and area 2 with A,B, & C in the backbone area 0. This is my plan, and I believe this will work and sort out my "discontiguous" problem, but can anybody confirm that i am right in what i am thinking and perhaps give me any advice on any potential problems that may arise.

Any advice will be appreciated.

Regards

Karl

[svermill]

Both EIGRP and OSPF support VLSM (athough not by default in the case of EIGRP). So if you move a remote network from one router to another at your central site, you must turn off autosummarization so that the discontiguous network can be found by others. This doesn't scale well in huge networks because of the added routes in the table (one for each subnet). But for smaller networks it is managable.

As for OSPF, I'm not sure if you will accomplish what you are hoping to. Area 0 needs it's own address space as do all other areas. And a backbone router will be backbone only or it will be a backbone router and participate in some other area(s). In order to connect out to your remote sites, at least one, but probably two, of your backbone routers will need to also be assigned to areas other than the backbone (unless your entire network is area 0 which probably isn't useful for what you are trying to accomplish).

At the ABRs you can summarize the contiguous networks of areas before advertising into the backbone. But if you move a network from one area to another and keep the discontiguous address assignment, you can no longer summarize (same as EIGRP or any other VLSM protocol). Now every network must be advertised individually. This can cause some serious SPF activity throughout the entire network because the state of each individual network will now need to be tracked throughout the network. One remote network goes down and/or comes up and the whole network (including other remote sites) recomputes the SPF tree.

So I would say that if you prefer not to readdress a network when moving it, you are actually better off with EIGRP. You always have to keep address assignments contiguous in order to properly summarize routes - regardless of the protocol.

[karl.jones]

Thanks for your comments, i guess what i am trying to achieve is a backnone of three routers in area 0, net 172.16.0.0/21, a bunch of remote routers in area 1, net 172.16.8 onwards, and a bunch of 172.22 routers in area 2. Basically, i need to be able to connect any router in area 1 or area 2 to any router in area 0. I could do this with eigrp because the network became discontiguous in parts, where as i think this may work in ospf. I need to be able to map any remote router to any router on the backbone.

Will this work ok,

Thanks for your comments.

[svermill]

Karl,

I think I understand your objective. You said "I could do this with EIGRP because the network became discontiguous." I presume you mean that you couldn't. If that is what you meant to say, then I suspect that you haven't turned off autosummarizatoin in EIGRP? Because EIGRP will support discontiguous networks - if configured to do so.

cisco#(config-router)no auto-summary

Yes, OSPF will also support discontiguous networks as you described above. Since I think you are intentionally planning to use discontiguous address space, you want to carefully consider how stable your remote connections are. Again, this is because of the SPF activity that can result in a non-summarized network.

If you are thinking that OSPF will allow you to connect to anywhere in area 0 without being discontiguous, I'm not sure that can be done.

Assume that router b is backbone only. Router a connects to area 1. Router c connects to area 2. Any remote router connected to the network obviously does so via routers a or c. If you have set up your address space hierarchically, you are summarizing addresses from the remotes into the backbone at routers a and c. Router b only sees routes to two networks (other than the backbone itself). The minute you move a remote router off of c and on to a (or vice versa), you have created a discontiguous network. This requires that you reconfigure the boundaries at which you summarize to something at least as specific as the mask of the network you moved from c to a.

Again, I think this lands you back where you started. EIGRP can do this too when autosummarization isn't turned on.

EIGRP example. You have router a connecting to networks which are subnets of 172.16.0.0 and router c connecting to networks which are subnets of 172.22.0.0. Now you move some remote network from router c and connect it to router a. Router a can still summarize for 172.16.0.0 (but it must be done manually at the interface level since autosummarization is turned off). It also now advertises, say, 172.22.1.0/24 that came over from router c. Router c can no longer summarize 172.22.0.0 to /16 because that would include 172.22.1.0/24.

Just remember that OSPF and EIGRP both advertise the subnet mask with the network and thus allow discontiguous configurations. EIGRP just doesn't do it by default.

[svermill]

Karl,

May I also suggest that you contact Vivek Baveja who is running the "Ask the Expert" for IP routing through the end of this week?

[karl.jones]

Hi Scott,

Thanks for your reply, i hear what you are saying and agree. I think what i was trying to do was create a 3 router wall between two networks but i guess this can be a problem. I thought OSPF would solve the problem allowing me to map the two areas right across all 3 routers but i guess i would still have the discontiguous problem. I am just trying to come up with the best solution before the wan grows to big. Whos Vivek Baveja? or who is ask the expert?.

Once again, thanks for your time

Merry Christmas

[svermill]

When you go to www.cisco.com/go/netpro (home page for this forum) you will see an "Ask the Expert" section of the page on the lower right. Click on "join the discussion." He is apparently on of Cisco's top IP routing experts. He has answered a bunch of questions like yours and he often includes references to white papers that he and others have written.

Good luck

Scott

[karl.jones]

thanks