H3C Technologies H3C WX3000 Series Unified Switches - Manual (Page 79)

10-6
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 bytes.
The maximum length of an Ethernet packet is 1500 bytes, that is, 0x05DC 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.
Packets with the value of the type or length field being in the range 0x05DD to 0x05FF are regarded as
illegal packets and thus discarded directly.
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:
z
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 10-6 802.3 raw encapsulation format
DA&SA(12) Length(2) Data
Currently, only the IPX protocol supports 802.3 raw encapsulation, featuring with the value of the two
bytes after the length field being 0xFFFF.
z
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 10-7 802.2 LLC encapsulation format
DA&SA(12) Length(2) Data DSAP(1) SSAP(1) Control(1)
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.
z
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 10-8 802.2 SNAP encapsulation format
DA&SA(12) Length(2) Data DSAP(1) SSAP(1) Control(1) OUI(3) PID(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.