This counter tracks received frames that were shorter than the minimum allowed Ethernet frame length. Undersized (runt) frames typically originate from a malfunctioning NIC, a collision fragment on a shared segment, or a misbehaving switch, so a climbing count here -- especially without corresponding collision activity -- points to a specific misbehaving device on the LAN rather than a link-quality issue. If this counter rises right after a particular device is connected to the switch feeding the CDD-564, that device's NIC or driver is the prime suspect for generating the runt frames.