Operation Manual - QoS&QoS Profile
Quidway S5600 Series Ethernet Switches
Chapter 1 QoS Configuration
Huawei Technologies Proprietary
1-4
Figure 1-2
An Ethernet frame with a 802.1Q tag header
As shown in the figure above, each host supporting 802.1Q protocol adds a 4-bit
802.1Q tag header after the source address of the former Ethernet frame header when
sending packets.
The 4-bit 802.1Q tag header contains a 2-bit Tag Protocol Identifier (TPID) whose value
is 8100 and a 2-bit 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.The 3 bits specify the precedence of the frame.8 classes of precedence are used to
determine which packet is sent preferentially when the switch is congested.
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 specification.
