H3C S5100-SI Operation Manual page 396

Figure 1-3 An Ethernet frame with an 802.1q tag header
As shown in Figure
header after the source address field of the former Ethernet frame header when sending
packets.
The 4-byte 802.1q tag header consists of a two-byte tag protocol identifier (TPID) field,
whose value is 0x8100, and a two-byte tag control information (TCI) field.
presents the format of the 802.1q tag header.
Figure 1-4 802.1q tag header
Byte 1
TPID (Tag protocol identifier)
1 0 0 0 0 0 0 1 0 0 0 0 0 0 0
7 6 5 4 3 2 1 0 7
In Figure 1-4, the three-bit priority field in TCI is 802.1p precedence (also known as CoS
precedence), which ranges from 0 to 7.
Table 1-4 Description on 802.1p precedence
802.1p precedence value
(decimal)
0
1
2
3
4
5
6
7
The precedence in the 802.1q tag header is called 802.1p precedence because its use is
defined in IEEE 802.1p.
3) Local precedence
1-3, each host supporting the 802.1q protocol adds a 4-byte 802.1q tag
Byte 2
0 Priority
6 5 4 3 2 1 0 7
802.1p precedence value
000
001
010
011
100
101
110
111
Byte 3
TCI (Tag control information)
CFI
VLAN ID
6 5 4 3 2 1 0 7 6
(binary)
1-7
Figure 1-4
Byte 4
5 4 3 2 1 0
Description
best-effort
background
spare
excellent-effort
controlled-load
video
voice
network-management