Priority Trust Mode - 3Com WX3000 Series Operation Manual

As shown in the figure above, each host supporting 802.1Q protocol adds a 4-byte 802.1Q tag header
after the source address of the former Ethernet frame header when sending packets.
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).
detailed contents of an 802.1Q tag header.
Figure 1-3 802.1Q tag headers
Byte 1
TPID (Tag Protocol Identifier)
1 0 0 0 0 0 0 1
7 6 5 4 3 2 1 0
In the figure above, the priority field (three bits in length) in TCI is 802.1p priority (also known as CoS
precedence), which ranges from 0 to 7.
Table 1-3 Description on 802.1p priority
802.1p priority (decimal)
0
1
2
3
4
5
6
7
The precedence is called 802.1p priority because the related applications of this precedence are
defined in detail in the 802.1p specifications.

Priority Trust Mode

A device can assign different types of precedence to the packets it receives as configured, such as
802.1p precedence, DSCP precedence, local precedence, and drop precedence.
Among the above-mentioned precedence types:
The local precedence is only of local significance. A local precedence corresponds to a specific
output queue. Packets with higher local precedence values take precedence over those with lower
precedence values and will be processed preferentially.
The drop precedence determines which packets are dropped preferentially. The higher the drop
precedence, the more likely a packet is dropped.
Byte 2
0 0 0 0 0 0 0 0 Priority
7 6 5 4 3 2 1 0 7
802.1p priority (binary)
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 5
1-5
Figure 1-3 describes the
Byte 4
4 3 2 1 0
Description
best-effort
background
spare
excellent-effort
controlled-load
video
voice
network-management