The GRE tunnel is disabled due to recursive routing, which occurs when the router learns the destination IP address for the tunnel interface through the tunnel itself. This creates a loop where the tunnel attempts to route traffic to its destination via the tunnel, causing it to collapse. The router then recalculates the routes, re-establishes the tunnel, and the cycle repeats. This is indicated by the system log message "%TUN-5-RECURDOWN Interface Tunnel 0 temporarily disabled due to recursive routing." References := Study CCNP: GRE Tunnels and Recursive Routing Problems