This tallies frames received on the ICL that the device could not classify into a recognized CICP frame type, which is generally an indicator of a protocol version mismatch or of something other than the intended peer chassis being bridged onto what should be a point-to-point inter-chassis link. A non-zero and growing count here would prompt an admin to check that both chassis are running compatible firmware versions and that nothing else is physically connected to the ICL cabling. For example, a spike to several hundred unknown frames right after a firmware upgrade on only one chassis would suggest a protocol-version incompatibility between the paired units.