Cisco 2950 Software Configuration Guide - Page 583

Chapter 30 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: Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Command configure terminal interface interface-id mls qos cos {default-cos | override} end show mls qos interface copy running-config startup-config Purpose Enter global configuration mode. Specify the interface to be trusted, and enter interface configuration mode. Valid interfaces include physical interfaces. 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. Return to privileged EXEC mode. Verify your entries. (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 Trusted Boundary In a typical network, you connect a Cisco IP Phone to a switch port as shown in Figure 30-4 on page 30-21, 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. 78-11380-10 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 30-23