GRE Tunnel.

[azibnaseem1]

Dear CISCO Team,

I have two ISP's setups both located on different cities. Yesterday, I have configured GRE Tunnel between them but neighbor relationship would't form between both ISP's routers. Neighbor is showing for moment into OSPF neighbor then gone and it happens continuously. Kindly, suggest me how could I successfully configure GRE tunnel. 

Thanks,

[Philip D'Ath]

The most common issue is where you advertise vi OSPF the end points of the tunnel, thorugh the tunnel itself, creating a recursive loop.

Can you post the tunnel configuration, physical interface configuration, static routes and OSPF config from one router please.

[azibnaseem1]

Dear Mr. Philip,

Thanks for the reply,

Requested outputs are attached from one side of my router. Kindly, review and update.

Thanks,

[Philip D'Ath]

You do have a recursive routing problem.  There is more than one solution.  I think this solution is the easiest.  You need to add a static host route for the remote "tunnel destination" on each router.

Add this to this router (and a similar thing for the remote router):

ip route 203.124.34.1 255.255.255.255 <default gateway>

[azibnaseem1]

Dear Mr. Philip,

Thanks for the reply,

IP address 203.124.34.1 is ping able from my router. What I want to do is that I have some private networks on both sides like 10.0.0.0/16. I do want to make these private network accessible and ping able from both sides. I did add static routes between both routers and my private networks are pinging from tunnel interface all other interfaces didn't able to ping my private networks. Could I accomplish this task using GRE?

Thanks,

[Richard Burts]

The original post talked about GRE tunnels and OSPF. Now the current post is talking about GRE and static routes. But there is no config information available to show what is there. So the only answer that I can give is that if the GRE tunnel is correctly configured and is in a stable operating state that it should be available for private networks on one side to access private networks on the other side using the GRE tunnel as transport. If you need help in figuring out why it is not working then provide details on what is configured.

HTH

Rick

[azibnaseem1]

Dear Mr. Richard,

Thanks for reply,

Tunnel interfaces are configured properly and router can ping their tunnel interfaces IP,s.

Any required configuration will be porvided.

Thanks,

[Philip D'Ath]

So to confirm; after adding the static routes I gave the GRE tunnels are now stable and staying up?

[azibnaseem1]

Dear Mr. Philip,

Tunnel Interface on both routers are UP and pinging each other whither I add static route or not.

tunnel Interfaces IP's: 172.16.10.1 - 2.

I can successfully ping from 172.16.10.1 to 172.168.10.2, but when I try to ping to 172.16.10.2 from my physical interface, its failed.

Thanks, 

[Richard Burts]

There are multiple things that might cause symptoms like you describe:

- there might be an access policy that prevents the ping from the physical interface.

- the GRE tunnel might be in a VRF that is different from the physical interface.

- there might be some route for 172.16.10.2 that points in the wrong direction.

Since you have provided no information about how the router is configured we can not know what the real cause is.

HTH

Rick

[paul driver]

Hello

The GRE destination addresses need to be reachable via the Physical interfaces not through OSPF peering between the GRE source/destination addressing

You may just have one OSPF peering at present via the GRE interfaces, thus the rtrs are seeing the tunnel destination addresses via this peering and not via the physical next hop of the GRE tunnels

Either advertise the physical next hop subnet, which would then create an additional OSPF peering but then the GRE destination would be advertised correctly through this new OSPF peering due to the shorter advertised OSPF interface cost

or

As suggested by Philip - add a static route for the gre destination pointing the physical next hop of each router.

res

Paul