What might cause an IP-IP tunnel to appear established but not transmit traffic?

An IP-IP tunnel can appear established, indicating that the endpoints have successfully configured the tunnel, but may not transmit traffic if there is no proper routing in place to direct packets into the tunnel. For traffic to be sent through the tunnel, there must be a route specified in the routing table that points to the tunnel interface for the destination IP addresses that should utilize the tunnel.

If a route into the tunnel is not present, the traffic destined for the target network or host will not know that it should be sent through the tunnel, leading to a situation where the tunnel is established but not actively transmitting any data. This routing configuration is critical because without it, the router won’t have any instructions on how to forward the packets, resulting in silence despite an established connection.

Other factors such as TTL settings, MTU configurations, or BGP relationships could impact traffic flow under different circumstances, but the fundamental need for a proper routing path into the tunnel makes the lack of a route the primary reason for the observed behavior of the tunnel appearing established yet not carrying traffic.