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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter20 Configuring QoS Configuring QoS
Configuring Tail-Drop Threshold Percentages
Tail drop is the default congestion-avoidance technique on Gigabit-ca pabl e E the rnet p orts. 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 thre sh old is no l onger e xcee ded. Ho we ver, packets
assigned to a second threshold continue to be queued and se nt a s long as the sec ond thr eshol d is no t
exceeded.
You modify the DSCP-to-threshold map to determine which DSCPs are mapped to which thres hold I D
by using the wrr-queue dscp-map interface configuration command. By default, all DSCP s are map ped
to threshold 1, and when this threshold is exceeded, all the packets are dropped.
If you use tail-drop thresholds, you cannot use WRED, an d v ice v er sa.
Beginning in privileged EXEC mode, follow these steps to configure the tail-drop threshold 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.
Step4 wrr-queue threshold queue-id
threshold-percentage1
threshold-percentage2
Configure tail-drop threshold percentages on each egress queue.
The default threshold is 100 percent for thresholds 1 and 2.
•For queue-id, specify the ID of the egress queue. The range is 1
to 4.
•For threshold-percentage1 threshold-percentage2, specify the
tail-drop threshold percentage values. Separate each value with a
space. The range is 1 to 100.
Step5 exit Return to global configuration mode.
Step6 interface interface-id Enter interface configuration mode, and specify the ingress
Gigabit-capable Ethernet interface.
Step7 wrr-queue dscp-map threshold-id dscp1
... dscp8 Map DSCP values to the tail-drop thresholds of the egress queues.
By default, all DSCP values are mapped to threshold 1.
•For threshold-id, specify the threshold ID of the queue. The range
is 1 to 2.
•For dscp1 ... dscp8, specify the DSCP values that are mapped to the
threshold ID. Enter up to eight DSCP values per command.
Separate each value with a space. The range is 0 to 63.
Step8 end Return to privileged EXEC mode.
Step9 show running-config
or
show mls qos interface interface-id
queueing
Verify the DSCP-to-threshold map.
Step10 show mls qos interface buffers Verify the thresholds.
Step11 copy running-config startup-config (Optional) Save your entries in the configuration file.