Allied Telesis AT-S62 - Manual (Page 194)

Chapter 14: Quality of Service
Section II: Advanced Operations 194
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 196.
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 196.
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:
❑ Strict priority
❑ Weighted round robin priority
Note
Scheduling is set at the switch level. You cannot set this on a per-
port basis.
Strict Priority Scheduling
With this type of scheduling, a port transmits all packets out of higher
priority queues before it transmits any from the low priority queues. For
instance, as long as there are packets in Q3 it does not handle any
packets in Q2.
The value to this type of scheduling is that high priority packets are
always handled before low priority packets.