Counts inbound packets discarded because their protocol field wasn't recognized or supported by the device. On a firewall this counter creeping up can indicate an unsupported or misconfigured protocol arriving on that link, for example an unusual EtherType frame hitting an interface not set up to parse it, which is a different problem than a simple policy-based drop. Example: ifInUnknownProtos rising right after a neighboring switch was reconfigured to trunk an extra VLAN suggests frames tagged with a protocol this interface doesn't parse are now arriving.