Allied Telesis AT-8100 Series - Manual (Page 1669)

AT-8100 Switch Command Line User’s Guide
1669
Configuring the Egress Queues
This section discusses a port’s egress queues, including how incoming
data are classified and marked according to priority and allocated to an
egress queue and, finally, how metering is applied. After the data packets
have been appropriately filtered, classified, and policed, they travel across
the switch’s internal paths carrying their assigned QoS tag markers—
DSCP, CoS, and bandwidth color. At the egress port, these markers are
read and used to determine which queue each data packet is forwarded
to.
There are eight egress queues allocated to each egress port. By default,
all queues on all ports are serviced in strict priority order. This means that
the highest numbered priority queue, queue 7, is emptied first. When
queue 7 is completely empty, the next highest priority queue, queue 6, is
processed. This process is continued until you reach queue 0. For a strict
priority queue to be processed, all higher priority queues must be empty.
In addition, you can configure the egress queues to the Weighted Round
Robin (WRR) scheduling method. With this method, you define the
number of packets transmitted from each queue before going on to the
next queue, so that each queue has the opportunity to transmit traffic. In
most instances, you give a greater weight to the higher priority queues.
For example, if you enable the WRR method and set the number of
packets transmitted from each queue to ten, the following scenario occurs.
First, queue 7 transmits ten packets, and then queue 6 transmits ten
packets. This is followed by queue 5 transmitting ten packets and
continues to queue 0. Then the process starts over with queue 7
transmitting ten packets. This process continues until all of the packets are
transmitted.
A second scenario occurs when the data packet transmitted is very small.
For example, a data packet that consists of 9 packets: The first queue, in
this case queue 7, is set to a weight of 15. When the 9 packets are
transmitted from queue, the transmission is completed. As a result, the
next data transmission is from queue 6.
The following subsections discuss how to set egress queues on a port and
how to do egress queue shaping on a port:
 “Determining the Egress Queues” on page 1670
 “Egress Queue Shaping” on page 1674