12-5
802.1p Priority
802.1p priority lies in Layer 2 packet headers and is applicable to occasions where Layer 3 header
analysis is not needed and QoS must be assured at Layer 2.
Figure 12-2 An Ethernet frame with an 802.1Q tag header
As shown in
Figure 12-2, 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 12-3 presents the format of the 802.1Q tag header. The Priority field in the 802.1Q tag header is
called the 802.1p priority, because its use is defined in IEEE 802.1p.
Table 12-6 presents the values for
802.1p priority.
Figure 12-3 802.1Q tag header
1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 Priority VLAN ID
TPID (Tag protocol identifier) TCI (Tag control information)
Byte 1 Byte 2
0
Byte 3 Byte 4
CFI
7 5432107 5432106 6 7 5432107 54321066
Table 12-6 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
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