This value holds the MAC address of the multicast group that most recently failed to register via GMRP on a given port; per the source, it is accessible only through the associated gmrpFailure notification rather than by direct polling. A network administrator reviewing a gmrpFailure trap uses this field to identify exactly which multicast group's registration attempt was rejected on that port. For example, this field appearing in a trap might carry the MAC address 01:00:5E:01:01:01, identifying the specific multicast group whose registration failed.