3Com 5500-EI PWR Install Manual page 670

Local precedence is a locally significant precedence that the device assigns to a packet. A
local precedence value corresponds to one of the eight hardware output queues. 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.
Priority trust mode
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.
1) For a packet carrying no 802.1q tag
When a packet carrying no 802.1q tag reaches the port of a switch, the switch uses the port
priority as the 802.1p precedence value of the received packet, searches for the local
precedence corresponding to the port priority of the receiving port in the 802.1p-to-local
precedence mapping table, and assigns the local precedence to the packet.
2) For an 802.1q tagged packet
When an 802.1q tagged packet reaches the port of a switch, you can use the priority trust
on the receiving port to configure the port to trust packet priority or use the priority
command on the receiving port to configure the port to trust port priority. By default, port
priority is trusted and the priority of a port is 0.
Trusting port priority
In this mode, the switch replaces the 802.1p priority of the received packet with the port
priority, searches for the local precedence corresponding to the port priority of the
receiving port in the 802.1p-to-local precedence mapping table, and assigns the local
precedence to the packet.
Trusting packet priority
In this mode, the switch searches for the local precedence corresponding to the 802.1p
priority of the packet in the 802.1p-to-local precedence mapping table and assigns the
local precedence to the packet.
Table 1-5 shows the default 802.1p priority-to-local precedence mapping table. You can
modify the default mapping tables at the CLI. For detailed configuration procedure, refer to
Configuring the Mapping between 802.1p Priority and Local
Table 1-5 802.1p priority-to-local precedence mapping table
802.1p priority
0
1
2
3
4
5
6
Local precedence
2
0
1
3
4
5
6
1-8
Precedence.