Administrator Guide
Class-of-Service 1501
Example 2: Long-Lived Congestion
The following example enables WRED discard for non-color-aware traffic.
Since a color-aware policer is not enabled, all traffic is treated as if it were
colored “green.” This means that only the “green” TCP and non-TCP WRED
thresholds are active. Since the default CoS queue is 1, this example is
suitable as a starting point for configuring WRED on a switch using the
default settings. Packets will be randomly dropped on an egress port when the
port becomes congested above the minimum threshold.
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 (about 300 jumbo frames). 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 (about 600 jumbo
frames).
If many (more than ¼) of the ports are becoming congested, then there is a
chance that the switch buffer capacity will be exceeded and the switch will
revert to tail-drop behavior. It is appropriate to lower the minimum threshold
when many ports are becoming congested and to raise the minimum
threshold when only one or two ports are becoming congested. Use the show
interfaces traffic command to display interface congestion.
1
Configure the green thresholds for TCP traffic on CoS queue 1. Other
thresholds are kept at their default values. The minimum threshold of
150% and maximum threshold of 200% with a drop probability of 2% are a
good starting point for tuning the WRED parameters for an enterprise
storage network that exhibits long term congestion on a few ports. Non-
TCP traffic is configured for tail-drop at the 100% threshold. No color-
aware processing is configured.
console(config)#random-detect queue-parms 1 min-thresh 150 30
20 100 max-thresh 200 90 80 100 drop-prob-scale 2 10 10 100
2
Enable WRED on cos-queue 1 (the default cos queue for packets marked
user priority 0).
console(config)#cos-queue random-detect 1