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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 20 Configuring QoS
QoS Configuration Examples
QoS Configuration for the Distribution Layer
This example focuses on the configuration steps for the Catalyst 3550-12G multilayer switch at the
distribution layer (see
Figure 20-10
). Because the classification was performed by the switches at the
edge of the network, fewer classification steps are needed at the distribution layer switch.
For the connection to the common wiring closet, Gigabit Ethernet interface 0/1 on the multilayer switch
is configured to trust the received CoS value. In this situation, the default CoS-to-DSCP map on the
multilayer switch is sufficient. For information on the default map settings, see the
“Configuring the
CoS-to-DSCP Map” section on page 20-39
.
For the connection to the intelligent wiring closet, Gigabit Ethernet interface 0/2 on the multilayer
switch is configured to trust the received DSCP value. The DSCP-to-threshold map also needs to be
configured on this ingress interface so that on the egress interface, WRED can provide congestion
avoidance control. By default, all DSCP values are mapped to threshold 1.
You need to configure several of the switch maps from their default settings. The object of the
configuration is to have only DSCP value 56 sent to the expedite queue (queue 4). The default
CoS-to-egress-queue map is sufficient; however, you need to configure the DSCP-to-CoS map so that
DSCP values 57 to 63 map to CoS 5.
For the egress interface, Gigabit Ethernet interface 0/5, WRR weights need to be configured by using
the wrr-queue bandwidth interface configuration command. WRED needs to be enabled and the
threshold percentages configured for each queue. The bandwidth allocated to each queue must be
configured to determine the ratio of the frequency at which packets are dropped from the queue.
Beginning in privileged EXEC mode, follow these steps to configure the multilayer switch at the
distribution layer:
Step 17
wrr-queue cos-map 4 6 7
Configure the CoS-to-egress-queue map so that CoS values 6
and 7 select queue 4 (this is the default setting).
Because the default DSCP-to-CoS map has DSCP values 56 to 63
mapped to CoS value 7, the matched traffic that is set to DSCP 56
goes to the queue 4, the priority queue.
Step 18
end
Return to privileged EXEC mode.
Step 19
show class-map videoclass
show policy-map videopolicy
show mls qos maps [cos-dscp | dscp-cos]
show mls qos interface queueing
Verify your entries.
Step 20
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Command
Purpose
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
mls qos
Enable QoS on the switch.
Step 3
interface gigabitethernet0/1
Enter interface configuration mode, and specify the ingress interface that
is connected to the common wiring closet.
Step 4
mls qos trust cos
Classify incoming packets on this port by using the packet CoS value.
