Configuring Tail-Drop Threshold Percentages - Cisco WS-C3550-12G Software Configuration Manual
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Chapter 20
Configuring QoS
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
Step 4
wrr-queue threshold queue-id
threshold-percentage1
threshold-percentage2
Step 5
exit
Step 6
interface interface-id
Step 7
wrr-queue dscp-map threshold-id dscp1
... dscp8
Step 8
end
Step 9
show running-config
or
show mls qos interface interface-id
queueing
Step 10
show mls qos interface buffers
Step 11
copy running-config startup-config
78-11194-03
Purpose
Enter global configuration mode.
Enable QoS on the switch.
Enter interface configuration mode, and specify the egress
Gigabit-capable Ethernet interface.
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.
Return to global configuration mode.
Enter interface configuration mode, and specify the ingress
Gigabit-capable Ethernet interface.
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.
Return to privileged EXEC mode.
Verify the DSCP-to-threshold map.
Verify the thresholds.
(Optional) Save your entries in the configuration file.
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring QoS
20-47
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