Allied Telesis AT-S63 - Manual (Page 225)

AT-S63 Management Software Menus Interface User’s Guide
Section II: Advanced Features 225
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-S63 management 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 Q7 it does not handle any
packets in Q6.
The value to this type of scheduling is that high priority packets are
always handled before low priority packets.
The problem with this scheduling method is that some low priority
packets might never be transmitted out the port because a port might
never get to its low priority queues. For instance, a port handling a large
volume of high priority traffic may be so busy transmitting that traffic
that it never has an opportunity to get to any packets that are stored in
its low priority queues.
Weighted Round Robin Priority Scheduling
The weighted round robin scheduling method is as its name implies. The
port transmits a set number of packets from each queue, in a round
robin fashion, so that each queue has a chance to transmit traffic. This
method guarantees that every queue receives some attention from the
port for transmitting packets.
To use this scheduling method, you need to specify the maximum
number of packets a port should transmit from a queue before moving
to the next queue. This is referred to as specifying the “weight” of a
queue. In all likelihood, you will want to give greater weight to the
packets in the higher priority queues over the lower queues.