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Catalyst 3560 Switch Software Configuration Guide
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Chapter 34 Configuring QoS
Configuring Standard QoS
Configuring the CoS Value for an Interface
QoS assigns the CoS value specified with the mls qos cos interface configuration command to untagged
frames received on trusted and untrusted ports.
Beginning in privileged EXEC mode, follow these steps to define the default CoS value of a port or to
assign the default CoS to all incoming packets on the port:
To return to the default setting, use the no mls qos cos {default-cos | override} interface configuration
command.
Configuring a Trusted Boundary to Ensure Port Security
In a typical network, you connect a Cisco IP Phone to a switch port, as shown in Figure 34-12 on
page 34-36, and cascade devices that generate data packets from the back of the telephone. The Cisco IP
Phone guarantees the voice quality through a shared data link by marking the CoS level of the voice
packets as high priority (CoS = 5) and by marking the data packets as low priority (CoS = 0). Traffic
sent from the telephone to the switch is typically marked with a tag that uses the IEEE 802.1Q header.
The header contains the VLAN information and the class of service (CoS) 3-bit field, which is the
priority of the packet.
For most Cisco IP Phone configurations, the traffic sent from the telephone to the switch should be
trusted to ensure that voice traffic is properly prioritized over other types of traffic in the network. By
using the mls qos trust cos interface configuration command, you configure the switch port to which
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 interface interface-id Specify the port to be configured, and enter interface configuration mode.
Valid interfaces include physical ports.
Step 3 mls qos cos {default-cos | override}Configure the default CoS value for the port.
• For default-cos, specify a default CoS value to be assigned to a port. If
the packet is untagged, the default CoS value becomes the packet CoS
value. The CoS range is 0 to 7. The default is 0.
• Use the override keyword to override the previously configured trust
state of the incoming packet and to apply the default port CoS value to
the port on all incoming packets. By default, CoS override is disabled.
Use the override keyword when all incoming packets on specified
ports deserve higher or lower priority than packets entering from other
ports. Even if a port was previously set to trust DSCP, CoS, or IP
precedence, this command overrides the previously configured trust
state, and all the incoming CoS values are assigned the default CoS
value configured with this command. If an incoming packet is tagged,
the CoS value of the packet is modified with the default CoS of the port
at the ingress port.
Step 4 end Return to privileged EXEC mode.
Step 5 show mls qos interface Verify your entries.
Step 6 copy running-config startup-config (Optional) Save your entries in the configuration file.