Riverstone Networks RS Switch Router User Guide Release 8.0 26-9
QoS Configuration
Weighted Random Early Detection (WRED)
The
exponential-weighting-constant
parameter specifies how fast the average queue size changes in response
to changes in the actual queue depth. In effect, the rate of change of the average queue size can be dampened. The
exponential-weighting-constant
accepts values from zero (0) to three (3) when WRED is applied to input
queues, and from zero (0) to seven (7) when WRED is applied to output queues. Note that 0 provides the least amount
of dampening, while larger numbers provide greater amounts of dampening.
This ability to dampen the response time of the average queue size changes WRED’s response to bursty traffic. For
example, notice in
Figure 26-1
that while the traffic (solid line) bursts at times, the average queue size (dotted curve)
is dampened such that it does not rise above the minimum threshold within the duration of the bursts. This keeps the
port from discriminating against traffic that might burst slightly at times.
Figure 26-1 Average queue size and bursty traffic
The
mark-prob-denominator
parameter is used to determine the probability of a packet being dropped when the
average queue size is between the minimum and maximum thresholds. The
mark-prob-denominator
accepts
values from zero (0) to three (3) when WRED is applied to input queues and from zero (0) to seven (7) when WRED
is applied to output queues. Note that the lower the value specified, the higher the probability that packets will be
dropped.
Both the
exponential-weighting-constant
value and the
mark-prob-denominator
value are somewhat
allegorical in the sense that neither of these values have a direct numerical significance other than acting as control
values for WRED. For example, if the value for
exponential-weighting-constant
is increased from 1 to 2, the
dampening of the average queue size response is not twice as slow. Because of this non-specific nature of
exponential-weighting-constant
and
mark-prob-denominator
and the fact that each network is different,
a discussion of recommend, specific settings for these values is beyond the scope of this User Guide.
When first implementing WRED on your RS, it is recommended to initially use the default values for min-threshold,
max-threshold, the weighting constant, and the probability denominator. If you begin or continue to experience
congestion, especially with TCP traffic, try adjusting WRED by making small changes in the various parameters
(one-at-a-time), and observing their effect on congestion.
To enable WRED on queues of specific input or output ports, enter the following command in Configure mode:
Enable WRED on input or output
queue of specified ports.
qos wred input|output
[
exponential-weighting-constant
<
num
>]
[
mark-prob-denominator
<
num
>] [
max-queue-threshold
<
num
>]
[
min-queue-threshold
<
num
>]
[
port
<port list>
| all-ports
]
[
queue
control | high | medium | low
]
min
th
max
th
Queue
Time
