Allied Telesis AT-S63 - Manual (Page 224)

Chapter 13: Quality of Service
224 Section II: Advanced Features
packets with a priority of 2 should be handled in Q0. The result is shown
in Table 9.
The procedure for changing the default mappings is found in ”Mapping
CoS Priorities to Egress Queues” on page 230. Note that because all ports
must use the same priority-to-egress queue mappings, these mappings
are applied at the switch level. They cannot be set on a per-port basis.
You can configure a port to completely ignore the priority levels in its
tagged packets and store all the packets in the same egress queue. For
instance, perhaps you decide that all tagged packets received on port 4
should be stored in the egress port’s Q3 egress queue, regardless of the
priority level in the packets themselves. The procedure for overriding
priority levels is explained in ”Configuring CoS” on page 227.
CoS relates primarily to tagged packets rather than untagged packets
because untagged packets do not contain a priority level. By default, all
untagged packets are placed in a port’s Q0 egress queue, the queue with
the lowest priority. But you can override this and instruct a port’s
untagged frames to be stored in a higher priority queue. The procedure
for this is also explained in ”Configuring CoS” on page 227.
Also note that the AT-S63 management software does not change the
priority level in a tagged packet. The packet leaves the switch with the
same priority it had when it entered. This is true even if you change the
default priority-to-egress queue mappings.
Scheduling A switch port needs a mechanism for knowing the order in which it
should handle the packets in its four egress queues. For example, if all
the queues contain packets, should the port transmit all packets from
Table 9. Example of Customized CoS Mappings to Priority Queues
IEEE 802.1p Priority Level AT-S63 Priority Queue
0 Q1
1 Q0
2 Q2
3 Q3
4 Q4
5 Q5
6 Q6
7 Q7