Tunnel probing refers to the mechanism where the denylisted tunnel or an end-point can be selected to serve only a single L2TP session initialization request. Only if this single L2TP session is successfully established over the selected tunnel, the tunnel can be removed from the denylist and consequently can serve new L2TP sessions. The tunnel is eligible for probing after its preconfigured time in the denylist has expired.
This behavior ensures that the new session initialization requests are not buffered while waiting for the tunnel to transition into operational state. Buffering would incur session setup delay and in the worst case it would cause session timeout if the L2TP tunnel cannot be established.
Without tunnel probing enabled, tunnels are automatically removed from the denylist upon the expiry of the preconfigured timer. New consecutive L2TP session initialization requests for such tunnels are always buffered.