Administrator Guide

Layer 2 Switching Commands 724
WRED Processing
WRED is intended to provide feedback to protocols (e.g. TCP) that depend
on packet loss to adjust their transmission rate. WRED drop behavior only
occurs when an interface is congested within the ranges specified. If
congestion exceeds the upper limit, queued 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.
Traffic ingressing the switch can be assigned to one of four drop precedences
based on a set of matching criteria. There are 3 drop precedences for TCP
traffic (green, yellow, and red) and one drop precedence for non-TCP traffic
(all colors). Users may configure the congestion thresholds at which packets
experiencing congestion are dropped randomly for each drop precedence and
may also configure the probability of a packet being dropped.
Packets are dropped at 100% when the egress queue size exceeds the
maximum value and at 0% when the queue size is below the minimum value.
Configuring a queue with a drop probability of 0% effectively applies tail-drop
behavior when the queue length exceeds the maximum threshold.
If the max-thresh parameter is less than the corresponding min-thresh
parameter, it is adjusted to be the min-thresh plus one.
For a given network, the minimum and maximum WRED thresholds should
be calculated to give a reasonable amount of buffering to TCP flows given the
switch buffer capacity. WRED thresholds are applied individually to each
physical interface. For the Dell NetworkingN2000/N3000 Series switches, a
threshold of 100% corresponds to a buffer occupancy of 295428 bytes queued
for transmission on an interface. For the N4000 Series switch, a threshold of
100% corresponds to a buffer occupancy of 666757 bytes queued for
transmission on an interface.
Use the classofservice dot1p-mapping command or the classofservice ip-
dscp-mapping command in conjunction with the classofservice trust
command to assign packets to a CoS queue based upon values contained
within the packet.
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:
2CSNXXX_SWUM204.book Page 724 Monday, January 25, 2016 1:25 PM