Configuring Tail-Drop Threshold Percentages - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 29 Configuring QoS
Command
Step 4 wrr-queue queue-limit weight1 weight2
weight3 weight4
Step 5
end
Step 6
show mls qos interface buffers
Step 7 copy running-config startup-config
To return to the default weights, use the no wrr-queue queue-limit interface configuration command.
This example shows how to configure the size ratio of the four queues. The ratio of the size allocated for
each queue is 1/10, 2/10, 3/10, and 4/10 for queues 1, 2, 3, and 4. (Queue 4 is four times larger than
queue 1, twice as large as queue 2, and 1.33 times as large as queue 3.)
Switch(config)# interface gigabitethernet0/1
Switch(config-if)# wrr-queue queue-limit 1 2 3 4

Configuring Tail-Drop Threshold Percentages

Tail drop is the default congestion-avoidance technique on Gigabit-capable Ethernet ports. With tail
drop, packets are queued until the thresholds are exceeded. For example, all packets with DSCPs
assigned to the first threshold are dropped until the threshold is no longer exceeded. However, packets
assigned to a second threshold continue to be queued and sent as long as the second threshold is not
exceeded.
You modify the DSCP-to-threshold map to determine which DSCPs are mapped to which threshold ID
by using the wrr-queue dscp-map interface configuration command. By default, all DSCPs are mapped
to threshold 1, and when this threshold is exceeded, all the packets are dropped.
If you use tail-drop thresholds, you cannot use WRED, and vice versa.
Beginning in privileged EXEC mode, follow these steps to configure the tail-drop threshold percentage
values on Gigabit-capable Ethernet ports:
Command
Step 1
configure terminal
Step 2 mls qos
Step 3 interface interface-id
78-11194-09
Purpose
Configure the egress queue size ratios.
The defaults weights are 25 (1/4 of the buffer size is allocated to each
queue).
For weight1, weight2, weight3, and weight4, specify a weight from 1
to 100. Separate each value with a space.
The relative size difference in the numbers show the relative
differences in the queue sizes.
When you enter this command, the queue is temporarily shutdown
during the hardware reconfiguration, and the switch drops newly
arrived packets to this queue.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
Purpose
Enter global configuration mode.
Enable QoS on the switch.
Enter interface configuration mode, and specify the egress
Gigabit-capable Ethernet interface.
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring Standard QoS
29-59