This counts frames that arrived with a CFM opcode field the device does not recognize as a valid message type such as CCM, LBM, LTM, or DMM, and were therefore dropped rather than handed to the matching processing function. Since the opcode is what tells the receiving MEP which CFM function should process the frame, a nonzero value here usually points to a firmware or protocol-version mismatch with whatever device sent the frame rather than a routine link problem. If this counter starts climbing right after a neighboring switch was upgraded to a newer Y.1731 message type this device's software does not yet decode, that is the kind of interop issue this counter surfaces.