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| 7.5 |  | Voice Quality Issues |
| 7.5.6 |  | Traffic classification at Layer 2 |

Cisco IP Phones can mark voice packets as high priority using CoS as well as ToS. By default, the phone sends 802.1Q tagged packets with the CoS and ToS set to a value of 5.

Because most PCs do not have an 802.1Q capable network interface card (NIC), they send the packets untagged without an 802.1Q field. Unless the applications running on the PC send packets with a specific CoS value, this field is zero. A special case is where the TCP/IP stack in the PC has been modified to send all packets with a ToS value other than zero. Typically this does not happen, and the ToS value is zero.

Even if the PC is sending tagged frames with a specific CoS value, Cisco IP Phones can zero out this value before sending the frames to the switch. This is the default behavior. Frames coming from the phone have a CoS of 5 and frames coming from the PC have a CoS of 0. When the switch receives these frames, it can take into account these values for further processing based on its capabilities.

The switch uses its queues (available on a per-port basis) to buffer incoming frames before sending them to the switching engine. (It is important to remember that input queuing comes into play only when there is congestion.) The switch uses the CoS value(s) to put the frames in appropriate queues. The switch can also employ mechanisms such as weighted random early detection (WRED) to make intelligent drops within a queue (also known as congestion avoidance) and weighted round-robin (WRR) to provide more bandwidth to some queues than to others (also known as congestion management).

There may be times, however, when it is desirable to trust the PC CoS (if sending tagged packets) or assign a value other than zero. This can be achieved on Catalyst switches as well.