The type/length field is 2 bytes in length. When used as a length field, the value in this field represents the number of bytes in the subsequent data field. This value does not include any bytes that might have been inserted in the padding field following the data field. The value of this field determines if it should be interpreted as a length as defined by the IEEE 802.3 standard or a type field as defined by the Ethernet protocol.
The maximum length of a data field is 1,500 bytes. Therefore, a value in this field that exceeds 1,500 ( 0x05DC ) indicates that a frame type rather than a length value is provided in this field. The IEEE 802.3 standard uses the value 1536 ( 0x0600 ) or greater to signal a type field. The AXI Ethernet Lite MAC does not perform any processing of the type/length field. This field is transmitted with the least significant bit first but with the high order byte first. This field is always provided in the packet data for transmissions and is always retained in the receive packet data.