This read-only counter tracks locally addressed PDUs that were successfully received but then discarded because their network-layer protocol was unrecognized or unsupported, such as something other than CLNP or ES-IS. An admin seeing this counter increase would suspect an unexpected or misconfigured protocol is being sent to this entity's NSAP address, worth investigating as either a network misconfiguration or unusual traffic. For example, a sudden rise in this counter might indicate a neighboring device is mistakenly sending a different OSI network-layer protocol to an address this entity expects only CLNP or ES-IS traffic on.