Scheduling – Allied Telesis AT-S82 User Manual

Page 51

AT-S82 Management Software User’s Guide

Table 1 lists the mappings between the eight CoS priority levels and the
four egress queues of a switch port.

For example, if a tagged packet with a priority level of 2 entered a port on
the switch, the switch would store the packet in Q11 on the egress port.

Note that priority 0 is mapped to CoS queue 1 instead of CoS queue 0
because tagged traffic that has never been prioritized has a VLAN tag user
priority of 0. If priority 0 was mapped to CoS queue 0, this default traffic
goes to the lowest queue, which is probably undesirable.

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 send 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-S82 management software uses weighted round-robin scheduling.
This method functions 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 an opportunity to transmit traffic. This method guarantees that
every queue receives some attention from the port for transmitting
packets.

To set up scheduling, 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. Table 2 provides a scheduling

Table 1. Default Mappings of IEEE 802.1p Priority Levels to Priority

Queues

IEEE 802.1p Priority Level

Port Priority Queue