This counts received frames shorter than the minimum valid Ethernet frame length. A rising count here typically indicates either a malfunctioning device on the segment sending malformed or runt frames, or a collision fragment being misinterpreted as a legitimate but undersized frame, distinct from CRC or alignment errors which involve otherwise correctly-sized frames. A specific device on the shared segment repeatedly sending undersized frames, perhaps a failing NIC elsewhere on the LAN, would show up as a steady rise in this counter on the modem receiving those runt frames.