I am trying to configure a GRE tunnel between two sites. In my configuration I have three Cisco routers. Router A and Router C are at the first site and are connected directly to each other - Router B is at the second site.
We can create the tunnel between Router A & B no problem. Running an show int tunnel 0 shows the link up. Router A can ping the interface on Router B and vice-versa. However I cannot ping the Router B interface from Router C. I feel like I am missing something basic but cannot identify it.
Relefant configuration and pings/traceroutes are available at the following URL.