GRE tunnel

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Nov 15th, 2009
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Hi,


I had configured GRE tunnel on both the routers. The tunnel is showing down now. Sometime if i shut and unshut it used to work, but now it's not coming up only.


Please let me know what could be the problem. The tunnel status is down now.

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Peter Paluch Sun, 11/15/2009 - 22:38
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  • Cisco Employee,

Hello,


A GRE tunnel is up if all these conditions are met:


1.) The interface with the source IP address (tunnel source) is up/up


2.) The IP address of the tunnel's destination (tunnel destination) is reachable according to the router's routing table


3.) If you are using GRE keepalives, they must be successfully received


Sometimes a tunnel can be flapping if the "tunnel destination" IP address is resolved through the tunnel interface itself. This can happen commonly if the route to the tunnel's destination is originally resolved by a default route and after the tunnel comes up, a dynamic routing protocol running over the tunnel advertises the actual network in which the tunnel's destination is located. As this network is more specific than the default route, it will be preferred to reach the tunnel's destination. However, as the information came in through the tunnel itself, a circular dependency is created and router will put the tunnel down with an error message that a recursive routing has occured. The solution in this case is quite trivial and consists of either inserting a static /32 routing entry towards the other tunnel endpoint in your routing table, or in making sure that the network in which the endpoint is located is never advertised in the routing protocol running over the tunnel.


So please verify these steps. If this does not help, be so kind to post the configuration of both tunnel interfaces and also the output of show ip interface brief and show ip route commands on both routers.


Best regards,

Peter


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