Allied Telesis AT-GS950/48, User Manual

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Chapter 15: Quality of Service and Class of Service
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The procedure for changing the default mappings is found in “Associate
Ports to CoS Priorities” on page 202. 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 also map an IPv6 packet header’s 8 bit priority field to one of the
switch’s priority queues. IPv6 traffic class priority settings are used by the
switch to differentiate between classes or priorities of IPv6 ports. The
procedure for mapping the IPv6 traffic class priorities is found in “IPv6
Traffic Class Mapping” on page 206.
One last thing to note is that the AT-S116 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.
Prioritizing
Untagged Packets
CoS relates primarily to tagged packets rather than untagged packets
because untagged packets do not contain a priority level. However, the
AT-GS950/48 switch has a priority associated with each individual ingress
port. By default, each port’s priority is Low. You can redefine this
parameter as described in “Associate Ports to CoS Priorities” on
page 202.
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 packets in queue Highest (the highest
priority queue) be processed through the switch before moving on to the
other queues, or should it instead just process a few packets from each
queue in a sequential fashion and, if so, how many?
This control mechanism is referred to as the scheduling algorithm .
Scheduling determines the order in which a port handles the packets in its
egress queues. The AT-S116 software has two types of scheduling:
 Strict priority
 Weighted round robin priority
To specify the scheduling, refer to “Associate Ports to CoS Priorities” on
page 202.
Note
Scheduling is set at the switch level. You cannot set this parameter
on a per-port basis.
Strict Priority Scheduling
With this type of scheduling, a port transmits all packets out of higher
priority queues before transmitting any from the lower priority queues. For