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Allied Telesis AT-S60 User Manual

Page 216

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Chapter 13: Class of Service

Section II: Advanced Features

216

Each switch port has two egress queues, low and high. When a tagged
frame enters a switch port, the switch responds by placing the frame
into one of the two egress queues according to following assignments:

IEEE 802.1p

AT-8400 Series Port

Priority Levels

Egress Queue

7

high

6

high

5

high

4

high

3

low

2

low

1

low

0

low

For example, a tagged frame with a priority tag of 6 is placed in the high
priority queue, while a frame with a priority tag of 1 is placed in the low
priority queue.

These priority-to-queue assignments can be overridden using the
AT-S60 management software on a per-port basis.

It should be noted that the determination of which egress queue a frame
is directed to is made when a frame is received on the ingress port and
before the frame is forwarded to the egress port. Consequently,
configuring this feature on a switch port influences the ingress frames.

For example, when you configure a switch port so that all ingress frames
with a priority level of 3 are handled by the high priority queue, all
frames with a priority level 3 that the port receives are directed to the low
priority egress queue of the egress port.

You can also use CoS to control which priority queue handles untagged
frames that ingress a port. By default, untagged frames (that is, frames
without VLAN or priority level information) are automatically assigned to
the low priority buffer. But you can configure CoS on a port so that all
untagged frames received on the port are directed to the high priority
queue on the egress ports.