H3C S7500 Series Operation Manual page 102

Operation Manual – VLAN
H3C S7500 Series Ethernet Switches
DA&SA(12)
Figure 1-5 802.2/802.3 encapsulation forma
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 brackets 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.
II. Encapsulation formats of 802.2/802.3 packets
802.2/802.3 packets are encapsulated in the following three formats:
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.
DA&SA(12)
Figure 1-6 802.3 raw encapsulation format
Only 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.
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.
DA&SA(12)
Figure 1-7 802.2 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.
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.
Length(2) DSAP(1)
Length(2)
Length(2) DSAP(1)
1-7
SSAP(1) Control ( 1) OUI(3)
t
Data
SSAP(1) Control ( 1)
Chapter 1 VLAN Overview
PID(2) Data
Data