Ethernet Framing

Distinguishing Between Ethernet Frame Types

1) Receive good frame.
2) Analyze frame. Perform the following steps, in order:


If the EtherType/Length value is greater than 0x05-DC (decimal 1500), then process the frame as Ethernet II. Any EtherType value greater than 0x05-DC (such as 0x0800 for IP or 0x81-37 for NetWare IPX/SPX) will be interpreted as an Ethernet II frame.

If the IPX header (0xFF-FF) follows the Length field, the frame is interpreted as a 802.3 (Raw) frame with Netware IPX/SPX traffic. Standard SSAP and DSAP values do not include hexadecimal FF, so the 802.3 (Raw) frame can be distinguished from LLC frames (Ethernet SNAP, 802.2).

Next, the byte following the length field (DSAP) is examined. If the value is 0xAA, the frame is interpreted as a SNAP frame. Otherwise, it is interpreted as a 802.2 frame.