This is the counterpart to the port-status counter, but for the Interface Status TLV, the optional CCM field a remote MEP uses to report whether its associated interface is up, down, dormant, or in another such state. An invalid value here means this device cannot trust the remote interface-state signal riding in that MEP's CCMs, which matters because some fault-correlation logic uses Interface Status to distinguish a physically down remote link from a failed remote MEP process. Persistent invalid Interface Status TLVs from one remote MEP, paired with otherwise clean CCM reception, suggests that remote device's interface encoding, not its connectivity, is what needs fixing.