H3C S3100-52P Operation Manual page 10

Operation Manual – VLAN
H3C S3100-52P Ethernet Switch
II. 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
Currently, only IPX 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
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.
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Chapter 1 VLAN Overview