1-6
The 4-byte 802.1q tag header consists of a two-byte tag protocol identifier (TPID) field, whose value is
0x8100, and a two-byte tag control information (TCI) field.
Figure 1-4 presents the format of the 802.1q
tag header.
Figure 1-4 802.1q 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
In
Figure 1-4, the three-bit priority field in TCI is 802.1p precedence (also known as CoS precedence),
which ranges from 0 to 7.
Table 1-4 Description on 802.1p precedence
802.1p precedence value
(decimal)
802.1p precedence value
(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 in the 802.1q tag header is called 802.1p precedence because its use is defined in
IEEE 802.1p.
3) Local precedence
Local precedence is a locally significant precedence that the switch assigns to a packet. A local
precedence value corresponds to one hardware output queue on the egress port. Packets with the
highest local precedence are processed preferentially. As local precedence is used only for internal
queuing, a packet does not carry it after leaving the queue.
4) Drop precedence
Drop precedence is used for making packet drop decisions. Packets with the highest drop precedence
are dropped preferentially.
Priority trust mode
A switch can assign different types of precedence to received packets as configured, such as 802.1p
precedence, DSCP values, local precedence, and drop precedence.
1) For an 802.1q-untagged packet
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