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.
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