I have some issue about GRE tunnel on router 3945 use c3900-universalk9-mz.SPA.151-4.M2 software by I create GRE tunnel connect to my site but it come up around 5 minute and line protocol is go down although I'm delete and recreate this interface but it has same issue. Below is GRE configuration detail:
H 147 182 A,U,U Tunnel169 (IP features on swidbs(7), IPv6 features on swidbs(10))
I'm sure on my router not enable ipv6 but I don't know why idb showing seem like this GRE has been enabled ipv6. It will be root cause of this case. Kindly everyone ever meet this issue please share how you are fix it.
You have keepalives configured on the tunnel interface. If it doesn't have an established tunnel on the other side, the tunnel interface will go down. If you want the tunnel interface up even if the tunnel isn't established, you can remove the keepalive from the interface and the tunnel interface will come up.