3Com WX3000 Series Operation Manual page 67

The switch identifies whether a packet is an Ethernet II packet or an 802.2/802.3 packet according to
the ranges of the two fields.
Extended encapsulation formats of 802.2/802.3 packets
802.2/802.3 packets have the following three extended encapsulation formats:
802.3 raw encapsulation: only the length field is encapsulated after the source and destination
address field, followed by the upper layer data. No other fields are included.
Figure 1-6 802.3 raw encapsulation format
DA&SA(12) Length(2)
Currently, only the IPX protocol supports 802.3 raw encapsulation, featuring with the value of the two
bytes after the length field being 0xFFFF.
802.2 logical link control (LLC) encapsulation: the length field, the destination service access point
(DSAP) field, the source service access point (SSAP) field and the control field are encapsulated
after the source and destination address field. The value of the control field is always 3.
Figure 1-7 802.2 LLC encapsulation format
DA&SA(12) Length(2)
The DSAP field and the SSAP field in the 802.2 LLC encapsulation are used to identify the upper layer
protocol. For example, if the two fields are both 0xE0, the upper layer protocol is IPX protocol.
802.2 sub-network access protocol (SNAP) encapsulation: encapsulates packets according to the
802.3 standard packet format, including the length, DSAP, SSAP, control, organizationally unique
identifier (OUI), and protocol-ID (PID) fields.
Figure 1-8 802.2 SNAP encapsulation format
DA&SA(12) Length(2)
In 802.2 SNAP encapsulation format, the values of the DSAP field and the SSAP field are always 0xAA,
and the value of the control field is always 3.
The switch differentiates between 802.2 LLC encapsulation and 802.2 SNAP encapsulation according
to the values of the DSAP field and the SSAP field.
When the OUI is 00-00-00 in 802.2 SNAP encapsulation, the PID field has the same meaning as the
type field in Ethernet II encapsulation, which both refer to globally unique protocol number. Such
encapsulation is also known as SNAP RFC1042 encapsulation, which is standard SNAP encapsulation.
The SNAP encapsulation mentioned in this chapter refers to SNAP RFC 1042 encapsulation.
Data
DSAP(1) SSAP(1) Control ( 1)
DSAP(1) SSAP(1) Control ( 1) OUI(3) PID(2)
1-6
Data
Data