Cisco 3560G 48TS Software Configuration Guide - Page 556

Configuring Standard QoS Chapter 28 Configuring QoS 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 28-11 on page 28-31, 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. With the trusted setting, you also can use the trusted boundary feature to prevent misuse of a high-priority queue if a user bypasses the telephone and connects the PC directly to the switch. Without trusted boundary, the CoS labels generated by the PC are trusted by the switch (because of the trusted CoS setting). By contrast, trusted boundary uses CDP to detect the presence of a Cisco IP Phone (such as the Cisco IP Phone 7910, 7935, 7940, and 7960) on a switch port. If the telephone is not detected, the trusted boundary feature disables the trusted setting on the switch port and prevents misuse of a high-priority queue. Note that the trusted boundary feature is not effective if the PC and Cisco IP Phone are connected to a hub that is connected to the switch. In some situations, you can prevent a PC connected to the Cisco IP Phone from taking advantage of a high-priority data queue. 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. Beginning in privileged EXEC mode, follow these steps to enable trusted boundary on a port: Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 Step 9 Command configure terminal cdp run interface interface-id cdp enable mls qos trust cos mls qos trust device cisco-phone end show mls qos interface copy running-config startup-config Purpose Enter global configuration mode. Enable CDP globally. By default, CDP is enabled. Specify the port connected to the Cisco IP Phone, and enter interface configuration mode. Valid interfaces include physical ports. Enable CDP on the port. By default, CDP is enabled. Configure the port to trust the CoS value in traffic received from the Cisco IP Phone. By default, the port is not trusted. Specify that the Cisco IP Phone is a trusted device. You cannot enable both trusted boundary and auto-QoS (auto qos voip interface configuration command) at the same time; they are mutually exclusive. Return to privileged EXEC mode. Verify your entries. (Optional) Save your entries in the configuration file. To disable the trusted boundary feature, use the no mls qos trust device interface configuration command. 28-34 Catalyst 3560 Switch Software Configuration Guide 78-16156-01