Cisco Systems 3750E manual Configuring a Trusted Boundary to Ensure Port Security, 36-41

Chapter 36 Configuring QoS

Configuring Standard QoS

	Command	Purpose
Step 3
	mls qos cos {default-cosoverride}	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.

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 36-15 on page 36-39, 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 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 the telephone is connected to trust the CoS labels of all traffic received on that port. Use the mls qos trust dscp interface configuration command to configure a routed port to which the telephone is connected to trust the DSCP labels of all traffic received on that port.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		36-41