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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 20 Configuring QoS
Configuring QoS
To return to the default thresholds, use the no wrr-queue threshold queue-id interface configuration
command. To return to the default DSCP-to-threshold map, use the no wrr-queue dscp-map
[threshold-id] interface configuration command.
This example shows how to configure the tail-drop queue threshold values for queue 1 to 10 percent
and 100 percent, for queue 2 to 40 percent and 100 percent, for queue 3 to 60 percent and 100 percent,
and for queue 4 to 80 percent and 100 percent on the egress interface (Gigabit Ethernet 0/1). The ingress
interface (Gigabit Ethernet 0/2) is configured to trust the DSCP in the incoming packets, to map
DSCPs 0, 8, 16, 24, 32, 40, 48, and 56 to threshold 1, and to map DSCPs 10, 20, 30, 40, 50, and 60 to
threshold 2:
Switch(config)# interface gigabitethernet0/1
Switch(config-if)# switchport mode access
Switch(config-if)# wrr-queue threshold 1 10 100
Switch(config-if)# wrr-queue threshold 2 40 100
Switch(config-if)# wrr-queue threshold 3 60 100
Switch(config-if)# wrr-queue threshold 4 80 100
Switch(config-if)# exit
Switch(config)# interface gigabitethernet0/2
Switch(config-if)# mls qos trust dscp
Switch(config-if)# wrr-queue dscp-map 1 0 8 16 24 32 40 48 56
Switch(config-if)# wrr-queue dscp-map 2 10 20 30 40 50 60
As a result of this configuration, when queue 1 is filled above 10 percent, packets with DSCPs 0, 8, 16,
24, 32, 40, 48, and 56 are dropped. The same packets are dropped when queue 2 is filled above 40
percent, queue 3 above 60 percent, and queue 4 above 80 percent. When the second threshold (100
percent) is exceeded, all queues drop packets with DSCPs 10, 20, 30, 40, 50, and 60.
Configuring WRED Drop Thresholds Percentages
WRED reduces the chances of tail drop by selectively dropping packets when the output interface begins
to show signs of congestion. By dropping some packets early rather than waiting until the queue is full,
WRED avoids dropping large numbers of packets at once.
All packets with DSCPs assigned to the first threshold are randomly dropped when the first threshold is
exceeded. However, packets with DSCPs assigned to the second threshold continue to be queued and
sent as long as the second threshold is not exceeded. Each threshold percentage represents where WRED
starts to randomly drop packets. By default, WRED is disabled.
Beginning in privileged EXEC mode, follow these steps to configure the WRED drop threshold
percentage values on Gigabit-capable Ethernet ports:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
mls qos
Enable QoS on the switch.
Step 3
interface interface-id
Enter interface configuration mode, and specify the egress
Gigabit-capable Ethernet interface.
