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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 26 Configuring QoS
Configuring 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 Trusted Boundary
In a typical network, you connect a Cisco IP phone to a switch port as shown in Figure 26-3 on
page 26-11. Traffic sent from the telephone to the switch is typically marked with a tag that uses the
802.1Q header. The header contains the VLAN information and the CoS 3-bit field, which determines
the priority of the packet. For most Cisco IP phone configurations, the traffic sent from the telephone to
the switch is 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 can configure the switch
port to which the telephone is connected to trust the CoS labels of all traffic received on that port.
In some situations, you also might connect a PC or workstation to the IP phone. In these cases, you can
use the switchport priority extend cos interface configuration command to configure the telephone
through the switch CLI to override the priority of the traffic received from the PC. With this command,
you can prevent a PC from taking advantage of a high-priority data queue.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Enter interface configuration mode, and specify the interface to be trusted.
Valid interfaces include physical interfaces.
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 port is CoS trusted and packets are untagged, the default CoS value
becomes the CoS value for the packet. 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 packets and to apply the default port CoS value to
all incoming packets. By default, CoS override is disabled.
Use the override keyword when all incoming packets on certain ports
deserve higher priority than packets entering from other ports. Even if
a port was previously set to trust DSCP, 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 egress 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.