H3C Technologies H3C S7500E Series Switches - Manual (Page 109)

15-3
Figure 15-2 An Ethernet frame with an 802.1Q tag header
As shown in Figure 15-2 , the 4-byte 802.1Q tag header consists of the tag protocol identifier (TPID,
two bytes in length), whose value is 0x8100, and the tag control information (TCI, two bytes in length).
Figure 15-3 presents the format of the 802.1Q tag header. The Priority field in the 802.1Q tag header is
called the 802.1p priority, because its use is defined in IEEE 802.1p. Table 15-3 presents the values for
802.1p priority.
Figure 15-3 802.1Q tag header
1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 Priority VLAN ID
TPID (Tag protocol identifier) TCI (Tag control information)
Byte 1 Byte 2
0
Byte 3 Byte 4
CFI
7 5 4 3 2 1 0 7 5 4 3 2 1 0 6 6 7 5 4 3 2 1 0 7 5 4 3 2 1 0 6 6
Table 15-3 Description on 802.1p priority
802.1p priority (decimal) 802.1p priority (binary) Description
0 000 best-effort
1 001 background
2 010 spare
3 011 excellent-effort
4 100 controlled-load
5 101 video
6 110 voice
7 111 network-management
EXP Values
The EXP field lies in MPLS labels and is used for QoS.