3Com 5500-EI PWR Install Manual page 79

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
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
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 RFC 1042 encapsulation, which is standard SNAP encapsulation.
The SNAP encapsulation mentioned in this chapter refers to SNAP RFC 1042 encapsulation.
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