3Com 4210 PWR Configuration Manual page 305

Priority Trust Mode
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 94 describes the detailed contents of an 802.1Q tag
header.
Figure 94 802.1Q tag headers
Byte 1
TPID (Tag Protocol Identifier)
1 0 0 0 0 0 0 1 0 0
7 6 5 4 3 2 1 0 7
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 222 Description of 802.1p priority
802.1p priority (decimal)
0
1
2
3
4
5
6
7
The precedence is called 802.1p priority because the related applications of this
precedence are defined in detail in the 802.1p specifications.
After a packet enters a switch, the switch sets the 802.1p priority and local
precedence for the packet according to its own capability and the corresponding
rules. The local precedence is locally significant precedence that the switch assigns
to the packet. It corresponds to an output queue. Packets with higher local
precedence values take precedence over those with lower precedence values and
will be processed preferentially.
By default, a Switch 4210 processes a received packet as follows:
For a packet without an 802.1q tag header, the switch uses the priority of the
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receiving port as the 802.1p precedence of the packet and looks up it in the
802.1p-precedence-to-local-precedence mapping table for the local
precedence, and then assigns the local precedence to the packet for it to be
added to a output queue.
For a packet with an 802.1q tag header, the switch replaces the 802.1p
■
precedence of the packet with the priority of the receiving port and looks up
the latter in the 802.1p-precedence-to-local-precedence mapping table for the
local precedence, and then assigns the local precedence to the packet for it to
be added to an output queue.
QoS Supported By Switch 4210 Family
Byte 2
0 0 0 0 0 0 Priority
6 5 4 3 2 1 0 7 6 5
802.1p priority (binary)
000
001
010
011
100
101
110
111
Byte 3 Byte 4
TCI (Tag Control Information)
cfi VLAN ID
4 3 2 1 0 7 6 5 4
Description
best-effort
background
spare
excellent-effort
controlled-load
video
voice
network-management
303
3 2 1 0