H3C H3C S5100-SI, Руководство по эксплуатации, Страница: 396 / 830

1-7
Figure 1-3 An Ethernet frame with an 802.1q tag header
As shown in Figure 1-3 , each host supporting the 802.1q protocol adds a 4-byte 802.1q tag
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. Figure 1-4
presents the format of the 802.1q tag header.
Figure 1-4 802.1q 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
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)
802.1p precedence value
(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
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