This read-only Counter32 counts received frames on a particular HDLC-like interface that had an invalid length, i.e., frames too short or too long to be valid per the HDLC framing rules. An admin monitors this counter to spot a peer or line that is delivering malformed frames, which typically points to interoperability or physical-layer issues rather than normal traffic. For example, a jump in this counter after a firmware upgrade on the remote end might indicate an HDLC framing incompatibility introduced by that upgrade.