This is the CCM-specific version of the STP-forwarding-state drop counter seen at the general Rx level: it counts CCMs specifically, not just any CFM frame, that were discarded because the receiving port's Spanning Tree state was not forwarding at the moment they arrived. Since CCMs are relied on as the heartbeat that declares a MEP down after roughly three and a half missed intervals, drops here during an STP reconvergence can transiently look like a real connectivity fault even though the link itself is fine. An admin correlating a short burst on this counter with an STP topology-change trap in the same window can rule out a genuine outage and confirm it was convergence-related instead.