Allied Telesis AT-S112, AT-GS950/16PS manual Scheduling, Prioritizing Untagged Packets

Chapter 15: Quality of Service and Cost of Service

Prioritizing Untagged Packets

Scheduling

The procedure for changing the default mappings is found in “Associate Ports to CoS Priorities” on page 182. 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.

One last thing to note is that the AT-S112 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.

CoS relates primarily to tagged packets rather than untagged packets because untagged packets do not contain a priority level. However, the AT-GS950/16PS switch has a priority associated with each individual ingress port. By default, each port’s priority is 0. You can redefine this parameter as described in “Associate Ports to CoS Priorities” on page 182.

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 Q3, the highest priority queue, be processed through the switch before moving on to the other queues, or should it instead just do 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-S112 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 182.

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 instance, as long as there are packets in Q3 it does not handle any packets in Q2. The value of this type of scheduling is that high priority packets are always handled before low priority packets which is required for voice or video data.

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