This 4-byte octet string captures the DXI2 header of the last SMDS PDU that triggered the 'Dxi2 Ctrl Invalid' violation, meaning the control field of the DXI level-2 header carried a value the switch doesn't recognize as valid for this circuit. An admin decoding these 4 bytes can inspect the exact bit pattern the CPE sent, which is useful when a device's DXI implementation is suspected of using a nonstandard or out-of-spec control field. For example, an older or third-party DXI adapter that doesn't fully conform to the SMDS DXI specification might repeatedly set an unrecognized control value, showing up here every time.