This read-only Counter counts frames received on this port that exceeded the maximum valid frame length and were therefore rejected. A nonzero and climbing count here on an otherwise stable port often points at an oversized-MTU mismatch with whatever's connected, for example an attached device configured for a larger frame size than ciscoTsPortCfgMaxTransmitUnit allows on this port. This is a concrete, checkable first cause to rule out before assuming a cabling or hardware fault.