OSPF Migration Path and Linux Routers

[mmc1800]

Hi,

I have had some limited experience with OSPF in the past (more BGP) and I am wondering if anyone has worked out a simple path to follow when migrating from statically routed networks to OSPF routing in the WISP scenario.

I am looking for something other than just turning on OSPF on all my routers and leaving all my customers down while I try to figure out the configurations.

Is it simple to change over one segment or link at a time to OSPF and leave the rest of the network statically routed and move out from the center, or from one end to the other? Are there any gotchas I should keep an eye out for while trying to make this change?

My network is still relatively small, so I think this would be a good time to get started on this before the job becomes larger (and managing the static routes is already becoming a PITA).

A good part of my static routing takes place on linux machines over various WAN interfaces and VPNs instead of StarOS boxes, so I am hoping that a vanilla linux Zebra OSPF will play nice with the StarOS versions. I haven't seen any mention of this and I am hoping that is a good thing, and I can't imagine why it would be, but never hurts to ask.

Basically what I am looking for OSPF to provide is both redundant failover/load balanced backhaul links between APs and back to the NOC over wireless links as well as redundant failover/load balanced routes out to the internet over a variety of LAN and WAN protocols. I route /24 private subnets to my customers and I am also hoping OSPF will help cut down the configuration time when I move a customer from one segment to another (though I think PPPOE/Radius may take care of this anyway which is also on my todo list). We will be lighting up our first set of APs shortly which introduce the opportunity for mutliple paths back to the NOC from other APs so this is now on the top of my list.

I am thinking OSPF will probably end up being better than trying to force my routed wireless links into bridged spanning tree configurations somehow which is how I handle these things when dealing with large ethernet networks (and OSPF looks like it will handle the load balancing when both links are up similar to ECMP which spanning tee doesn't).

I already have multiple physically diverse internet connections serving the network (segmenting VOIP from general internet traffic for example) and I am using static linux metric routing and ECMP routes on linux machines to handle this but I would like to spread OSPF across the entire network for obvious reasons.

Any information or ideas regarding experiences in switching over to OSPF with the least possible downtime and simple backouts if something fails would be greatly appreciated.

[bminish]

Well, it might not be a migration path per se :-) but when we started playing around with this we noticed that static routes take preference so you can configure your OSPF setup then late one night you can go in and delete the static routes (and break everything..) on a node by node basis.

.Brendan

[mmc1800]

That is the kind of thing I was looking for. I was not real happy about having to enable OSPF to even look at the configuration heh....

Good to know the static routing will keep on keepin on...

[oscarBravo]

One other point on this: the default 'factory preset' OSPF configuration doesn't have any 'network' statements in it, so it won't advertise any routes when you first turn it on. It's probably a good idea to enable OSPF, then configure everything except the 'network' statements. When the config looks good, add the 'network' statements.

Once the routers at both ends of a link are reporting stable OSPF links, you can remove the static routes and save/apply changes. This should give a pretty stable migration path on a link-by-link basis.

[mmc1800]

Just following up on this for anyone who is interested in this kind of migration from static to OSPF routing.

First off in case you miss it anywhere else, enabling OSPF and putting the entire configuration in place does not mess with your static routes. The static routing takes priority over the OSPF routes so you can go ahead and leave everything in place with static routes and get OSPF all working and then start to remove the static routes.


Probably a small tutorial on OSPF from inside staros would be nice to have for people who are new to it, but really the online documentation for OSPF from Cisco etc. is very good. I used this page often when trying to figure out what various things would do:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfospf.htm#wp1001936 (http://http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfospf.htm#wp1001936)

Also, the people in these forums (the saga of OscarBravo and Bminish especially) are really helpful in these forums and the info in this thread was very helpful (3537 if the links dont work):

http://forums.star-os.com/showthread.php?t=3537

I have the majority of my routers now with no default routes and everything seems to be working well.

You can disable a static route and apply changes and see if the OSPF route works, and if it doesn't you can enable the static route again. All in all it was pretty painless to get this working.

The only gotcha I could imagine is that you need to make sure you can access any remote radios you are working on from an ssh terminal from a machine on the same subnet in case you break your routing. If you turn off your default route on a box and it no longer has a route back to the terminal you are working on you could be SOL but this was never a problem for me as I would ssh into the radio from the system pull down menu on a directly connected staros box with a /30 subnet (no routes needed).

There is also a strange interface glitch if you open the advanced routing dialog when you have more than one SSH client connected to the box at a time, that can make it very confusing to tell if you have OSPF off or on. So I would suggest just using one connection at a time while configuring OSPF.

Thanks for all the help, I have a couple questions still (going to start a new thread) but all in all OSPF is working and seems stable so far.