Allied Telesis AT-S62 - Manual (Page 291)

AT-S62 Menus Interface User’s Guide
Section II: Advanced Operations 291
The procedure for changing the default mappings is found in Mapping
CoS Priorities to Egress Queues on page 297. 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 an 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 294.
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 294.
One last thing to note is that the AT-S62 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
Q3, the highest priority queue, before moving on to the other queues, or
should it instead just do a few packets from each queue and, if so, how
many?
This control mechanism is called scheduling. Scheduling determines the
order in which a port handles the packets in its egress queues. The
AT-S62 software has two types of scheduling:
3 Q1
4 Q2
5 Q3
6 Q3
7 Q3
Table 8 Customized Mappings of IEEE 802.1p Priority Levels to Priority
Queues
IEEE 802.1p Priority
Level
Port Priority Queue