Allied Telesis AT-S63 - Manual (Page 223)

AT-S63 Management Software Menus Interface User’s Guide
Section II: Advanced Features 223
Each switch port has eight egress queues. The queues are Q0 through
Q7. Q0 is the lowest priority queue and Q7 is the highest. A packet in a
high priority egress queue is typically transmitted out a port sooner than
a packet in a low priority queue.
Table 8 lists the mappings between the eight CoS priority levels and the
eight egress queues of a switch port.
For example, assume that a tagged packet with a priority level of 3
enters a port on the switch. The switch, after examining the packet’s
destination address, determines that the packet is to be sent out port 6.
The switch must now determine which of port 6’s egress queues the
packet should be stored in. It examines the priority level in the packet,
which is 3. Now the switch knows to store the packet in port 6’s Q3
egress queue.
Note that priority 0 is mapped to CoS queue 1 instead of CoS queue 0
because tagged traffic that has never been prioritized has a VLAN tag
User Priority of 0. If priority 0 was mapped to CoS queue 0, this default
traffic goes to the lowest queue, which is probably undesirable. This
mapping also makes it possible to give some traffic a lower priority than
the default traffic.
You can change these mappings. For example, you might decide that
packets with a priority of 5 need to be handled by egress queue Q7 and
Table 8. Default Mappings of IEEE 802.1p Priority Levels to Priority Queues
IEEE 802.1p Priority Level Port Priority Queue
0 Q1
1 Q0
2 Q2
3 Q3
4 Q4
5 Q5
6 Q6
7 Q7