H3C Technologies H3C S3100 Series Switches - Manual (Page 397)

Operation Manual – QoS
H3C S3100-52P Ethernet Switch Chapter 1 QoS Configuration
1-4
Figure 1-2
An Ethernet frame with an 802.1Q tag header
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 contains a 2-byte Tag Protocol Identifier (TPID) whose
value is 8100 and a 2-byte Tag Control Information (TCI). TPID is a new class defined
by IEEE to indicate a packet with an 802.1Q tag. Figure 1-3 describes the detailed
contents of an 802.1Q tag header.
Figure 1-3
802.1Q tag headers
In the figure above, the 3-bit priority field in TCI is 802.1p priority in the range of 0 to 7.
These three bits specify the precedence of the frame. Eight classes of precedence are
used to determine which packet is sent preferentially when congestion occurs.
Table 1-3
Description on 802.1p priority
CoS (decimal) CoS (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.