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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter20 Co nf iguring 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 queu e thre sho ld v alu es for q ue ue 1 t o 1 0 perc en t
and 100 percent, for queue 2 to 40 percent and 100 perc ent , for qu eu e 3 to 60 per cent an d 100 p er cent ,
and for queue 4 to 80 percent and 100 percent on the e gress 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 DSC Ps 10 , 20, 3 0, 40 , 5 0, a nd 60.
Configuring WRED Drop Thresholds Percentages
WRED reduces the chances of tail drop by selectively dropping packets when the outp ut 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 t hreshold percentage represents where WRED
starts to randomly drop packets. By default, WRED is disa bled .
Beginning in privileged EXEC mode, follow these steps to confi gure t he WR ED drop t hres hold
percentage values on Gigabit-capable Ethernet ports:
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 mls qos Enable QoS on the switch.
Step3 interface interface-id Enter interface configuration mode, and specify the egress
Gigabit-capable Ethernet interface.