1-7
2) 802.1p priority
802.1p priority lies in Layer 2 packet headers and is applicable to occasions where the
Layer 3 packet header does not need analysis but QoS must be assured at Layer 2.
Figure 1-3 An Ethernet frame with an 802.1Q tag header
As shown in the figure above, 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 1-4 describes the detailed contents of an
802.1Q tag header.
Figure 1-4 802.1Q tag headers
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 1-4 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.
3) Local precedence
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