This counts instances where the destination address value itself in a SIP3 PDU header was invalid, as opposed to just its type tracked separately by lportSmdsCntSip3DaTypeInvalids, meaning the address does not conform to the expected SMDS address format. An admin treats a rising count here as evidence of address corruption or a sender generating malformed destination addresses, which would cause those PDUs to be undeliverable regardless of any other correctness. For example, if this counter rises for traffic from one specific source while other sources on the same trunk show none, that one source's SMDS stack is likely the culprit generating bad destination addresses.