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AT-S81 Management Software User’s Guide

Section I: Using the Menus Interface 135

For example, a tagged packet with a priority tag of 6 is placed in the

egress port’s highest priority queue of 3, while a packet with a priority tag

of 1 is placed in the lowest priority queue.

Note

QoS is disabled by default on the switch.

You can customize these priority-to-queue assignments using the AT-S81

management software. The procedure for changing the default mappings

is found in “Mapping CoS Priorities to Egress Queues” on page 137.

You can configure a port to completely ignore the priority levels in its

tagged packets and instead use a temporary priority level assigned to the

port. For instance, perhaps you decide that all tagged packets received on

port 4 should be assigned a priority level of 5, regardless of the priority

level in the packets themselves. The procedure for overriding priority

levels is explained in “Configuring CoS” on page 140.

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 140.

One last thing to note is that CoS 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.

Table 3. Default Mappings of IEEE 802.1p Priority Levels

to Egress Port Priority Queues

IEEE 802.1p Traffic Class

Egress Port Priority

Queue

1 2 ... 130 131 132 133 134 135 136 137 138 139 140 ... 359 360

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