Administrator Guide

1540 Differentiated Services
WRED
WRED Processing
Traffic ingressing the switch can be assigned to one of four drop probabilities
based on a set of matching criteria. There are three drop probabilities for TCP
traffic (green, yellow, and red) and one drop probability for non-TCP traffic
(all colors). Users may configure the congestion thresholds at which packets
queued for transmission are dropped for each color.
WRED is intended to provide early feedback to protocols (e.g., TCP) that
depend on packet drop to adjust their transmission rate. WRED packet drops
only occur when the system is congested within the ranges specified. If
congestion exceeds the upper limit, packets will be dropped at the rate of
traffic ingressing the system, e.g., 100%. If the congestion is less than the
lower limit, no packets will be dropped.
WRED Drop Probabilities
Between the minimum and maximum thresholds, the drop probability is
divided into eight discrete levels of increasing probability of packet drop. The
levels are as follows:
0 – 6.25% of maximum drop probability
1 – 18.75% of maximum drop probability
2 – 30.25% of maximum drop probability
3 – 43.75% of maximum drop probability
4 – 56.25% of maximum drop probability
5 – 68.75% of maximum drop probability
6 – 81.25% of maximum drop probability
7 – 92.75% of maximum drop probability
As an example, with a drop probability of 50%, a minimum threshold of 10%
and a maximum threshold of 90%, the drop probability from 10% to 20%
congestion is 3.125%, from 21% to 30% congestion is 9.375%.
NOTE: SRED is not supported on the Dell EMC Networking N1500 Series switch.