Administrator Guide

1492 Class-of-Service
Exponential Weighting Constant
The degree of congestion is determined by sampling the egress queue depth
and calculating an average queue size. The exponential weighting constant
smooths the result of the average queue depth calculation by the function:
average depth = (previous queue depth * (1-1/2^n)) + (current queue depth * 1/2^n)
The average queue depth is used to select the drop probability for packets
queued for egress. Because the instantaneous queue depth fluctuates rapidly,
larger values of the weighting constant will cause the average queue depth
value to respond to changes more slowly than smaller values.
WRED Color-Aware Processing
Packets may be assigned to different colors using an ingress policing policy.
Each color has a different profile of WRED drop probabilities. This capability
allows the operator to configure different WRED drop policies based on the
incoming packet's CoS, secondary CoS, IP DSCP, or IP precedence values.
To assign a color to incoming packets, define a class map with the desired
matching criteria, assign the class map to a policy map, and set a metering
rule in the policy map. Dell Networking switches implement three policing
meters: a simple two-color meter, a single-rate three-color meter, and a two-
rate three-color meter.
Each of these may be configured in color-aware or color-blind mode.
The conform-color command is used to enable color-aware mode. If the
conform-color command is not used, packets are processed in color-blind
mode. Color-blind mode means that all packets are initially colored green and
are assigned their final color by the meter. Color-aware mode means that
packets are pre-colored prior to entering the meter that assigns the final
packet color. Packets that match the conform-color class are pre-colored
green.
Packets that match the exceed color class are pre-colored yellow. Packets that
do not match the conform color or exceed color class are pre-colored red, that
is, they will always have a final color of red and will be processed accordingly.
• Packets that are pre-colored green and exceed the CIR will be colored
yellow. Those that exceed the PIR will be colored red.