1-5
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).
Figure 1-3 describes the
detailed contents of an 802.1Q tag header.
Figure 1-3 802.1Q tag headers
Byte 1
TPID (Tag Protocol Identifier) TCI (Tag Control Information)
Priority cfi VLAN ID
Byte 2Byte 3Byte 4
00 0 00 0 001 0 0 00 10 0
3274 1065 3 274 1065 3274 1065 3274 1065
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) 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
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:
z 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.
z The drop precedence determines which packets are dropped preferentially. The higher the drop
precedence, the more likely a packet is dropped.
