QoS and QoS Policies
154
Quality of Service Guide
3HE 11014 AAAC TQZZA Edition: 01
Referring to Figure 24, one WRED slope curve can manage discards on high-priority
traffic and another WRED slope curve can manage discards on low-priority traffic.
The minT, maxT and maxDP values configured for high-priority and low-priority traffic
can be different and can start discarding traffic at different thresholds. Use the
start-avr, max-avr, and max-prob commands to set the minThreshold,
maxThreshold, and maxDProbabilty values, respectively.
Figure 24 WRED for High-Priority and Low-Priority Traffic on the Same
Queue
The formula to calculate the average queue size is:
average queue size = (previous average × (1 – 1/2^TAF)) + (current queue size ×
1/2^TAF)
The Time Average Factor (TAF) is the exponential weight factor used in calculating
the average queue size. The time_average_factor parameter is not
user-configurable and is set to a system-wide default value of 3. By locking TAF to a
static value of 3, the average queue size closely tracks the current queue size so that
WRED can respond quickly to long queues.
Note: The figure shows a step function at maxT. The maxDP value is the target value
entered for the configuration and it partly determines the slope of the weighting curve. At
maxT, if the arrival of a new packet will overflow the buffer, the discard probability jumps
to 1, which is not the maxDP value. Therefore, a step function exists in this graph.
MaxDP
MinT MaxT
1
WRED low_prio
MaxDP
MinT MaxT
1
WRED hi_prio
CoS-7
19696
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