Add EIGRP to GRE Tunnel, address question

[j.vong]

Hello, I would like to ask if my GRE Tunnel interface has a "ip unnumbered Fa0/0" where my Fa0/0 is a 10.x address. And my tunnel interface has defined a tunnel source address of 150.1.1.1 and destination of 170.1.1.1

My question is when I add the EIGRP to the router (the current router is running static route and we want to delete the static route and add EIGRP as there are multiple tunnels in this router and want it to learn dynamic to take the best path).

So

router eigrp 1

network 10.0.0.0

network 150.1.0.0

network 170.1.0.0

......

the question is should I only need to have 10.0.0.0 statement, no need to add 150 and 170 under EIGRP?

Thank you very much for your assistance.

[thisisshanky]

You should be fine with 10.0.0.0 network under eigrp. Adding 150 and 170 networks will enable EIGRP on those interfaces which have an ip address falling on those networks.

[j.vong]

Hello Shanky,

I have tried the other day and it doesn't work. Not due to the 150 or 170.

I have 3 GRE tunnels defined in this site going to 3 differnt locations over the same serial interface out to a layer 3 IP network.

All 3 tunnel interfaces are defined with the same "ip unnumbered Fa0/0". Each has the same tunnel source address of 150.x with a different tunnel destination address of each of the 3 destination.

After adding eigrp with network 10.0.0.0 to the site, the tunnel keeps going up and down and up and down when I show log.

When I put in sh ip eigrp topology on the site, it sometimes learns a lot of routes in the table, and next few seconds later, the routes disappears again, (I guess that due to the tunnel keeps going up and down).

Someone said it is caused by all 3 tunnels shared the same Fa0/0 address and causes the Eigrp to disable the tunnel. I'm not sure if this is the reason.

Can you help?

Thank you very much

[thisisshanky]

When yo usay the tunnel keeps going up and down, does it mean, the line protocol status of tunnel interface go up and down ? If thats the case, before enabling EIGRP, see if you can get the tunnel interface status up/up. Once you do that, try enabling EIGRP. Since the tunnels are going to different destinations (Are they physically seperate routers at the destinations?) I dont think the same source address (unnumbered fa0/0) should matter..

EIGRP could be flapping, because the tunnel interfaces is not stable.

[j.vong]

Hello Shanky,

Yes, the line protocol says down and then up and then down.

The existing router just have static routes and all the tunnels are up/up without any flapping issue. The time when it becomes flapping is after I add EIGRP to it and it starts getting the problem. Now I remove the EIGRP and put back the static routes, it's been up/up ever since.

The three destinations are all physically separate routers where two of them are in HSRP mode.

Somehow not all 3 tunnels are flapping, just one particular one doing that. And that tunnel on the destination side has a keepalive 3 10 command on it but on the same tunnel on my main site does not have the keepalive command on it. Can this cause the problem?

Any suggestion?

Thanks.

[Richard Burts · Accepted solution]

I have run EIGRP over several unnumbered interfaces (in the past) and I do not believe that this causes your problem.

On most media if keepalives are mismatched (one end does configure keepalive and the other end does not) it will certainly cause problems. But usually it results in one end being up/down and not flapping. My reading of the documentation for GRE keepalive (which is a very new capability of IOS) is that it does not need to match between ends of the GRE tunnel. So while I think it might be nice to have both ends match about keepalive I do not think that this is the cause of your problem.

I believe that your problem is probably due to recursive routing. This can be an issue with GRE tunnels and dynamic routing protocols. The problem occurs when the destination address of the tunnel appears in the routing table as being reachable through the tunnel (as if you go through the tunnel to get to the tunnel destination). For GRE tunnels to work properly, the router must have a route to the tunnel destination that does not depend on the tunnel itself. And recursive routing on the tunnel destination will frequently produce the flapping synptoms that you describe.

So I suggest that you look carefully at the router(s) where the tunnel is flapping and be sure that EIGRP is not advertising the tunnel destination as an EIGRP route through the tunnel.

HTH

Rick

[j.vong]

Hello Rick,

Your suggestion on recursive routing makes a lot of sense. I think this is exactly the problem.

Thank you very much for your help.

Hello Shanky,

Also thank you for your help as well.