Administrator Guide

1476 Class-of-Service
• Weighted Random Early Detection (WRED)—Drops packets queued for
transmission on an interface selectively based their drop precedence level.
For each of four drop precedence levels on each WRED-enabled interface
queue, the following parameters can be configured:
– Minimum Threshold: A percentage of the interface queue size below
which no packets of the selected drop precedence level are dropped.
– Maximum Threshold: A percentage of the interface queue size above
which all packets of the selected drop precedence level are dropped.
– Drop Probability: When the queue depth is between the minimum
and maximum thresholds, this value provides a scaling factor for
increasing the number of packets of the selected drop precedence level
that are dropped as the queue depth increases. The drop probability
supports configuration in the range of 0 to 10%, and the discrete
values 25%, 50%, and 75%. Values not listed are truncated to the next
lower value in hardware.
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 Networking N2000/N3000 Series switches, a threshold
of 100% corresponds to a buffer occupancy of 295428 bytes queued for
transmission on an interface. For the Dell Networking N4000 Series switch, a
threshold of 100% corresponds to a buffer occupancy of 666757 bytes queued
for transmission on an interface.
CoS Queue Usage
CoS queue 7 is reserved by the system and is not assignable. It is generally
recommended that the administrator utilize CoS queues 0 to 3, as CoS
queues 4-6 may be used by the system for other types of system traffic, for
example, routing protocol PDU handling.
NOTE: WRED is not supported on the Dell Networking N1500 Series switch.