3Com Switch 7754, Configuration Manual

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92 C
HAPTER
10: VLAN O
VERVIEW
In the two figures, DA and SA refer to the destination MAC address and source
MAC address of the packet respectively. The number in the bracket indicates the
field length in bits.
The maximum length of an Ethernet packet is 1500 bytes, that is, 5DC in
hexadecimal, so the length field in 802.2/802.3 encapsulation is in the range of
0x0000 to 0x05DC.
Whereas, the type field in Ethernet II encapsulation is in the range of 0x0600 to
0xFFFF.
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.
Encapsulation formats of 802.2/802.3 packets
802.2/802.3 packets are encapsulated in the following three formats:
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802.3 raw encapsulation: only the length field is encapsulated after the source
and destination address field, followed by the upper layer data. The type field is
not included.
Figure 30 802.3 raw encapsulation format
Only the IPX protocol supports 802.3 raw encapsulation format currently. This
format is identified by the two bytes whose value is 0xFFFF after the length field.
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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.
Figure 31 802.2 LLC encapsulation format
The DSAP field and the SSAP field in the LLC part are used to identify the upper
layer protocol. For example, the two fields are both 0xE0, meaning that the upper
layer protocol is IPX protocol.
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802.2 sub-network access protocol (SNAP) encapsulation: the length field, the
DSAP filed, the SSAP field, the control field, the OUI field and the PID field are
encapsulated according to 802.2/802.3 standard packets.
Figure 32 802.2 SNAP encapsulation format
In 802.2 SNAP encapsulation format, the values of the DSAP field and the SSAP
field are always AA, and the value of the control field is always 3.
DA&SA(12) Length(2) Data
DA&SA(12) Length(2) Data DSAP(1) SSAP(1) Control(1)
DA&SA(12) Length(2) Data DSAP(1) SSAP(1) Control(1) OUI(3) PID(2)