Exam 350-801 topic 1 question 394 discussion

Not sure why B

The phone is trusted and the phone will mark the packets

"we won’t remark any packets or Ethernet frames anymore at the access layer switch. The IP phone will mark all traffic. Note that the computer is outside of the QoS trust boundary. This means that we don’t trust the marking of the computer. We can remark all its traffic on the IP phone if we want."

1. That is correct that there is no CoS field if the phone is receiving untagged frames from the PC. If the frames are tagged, the CoS value will be reset to zero. 2. Yes. The DSCP of traffic received from the PC is also re-marked to zero by default. Therefore, 'trust dscp' should be fine on your switch port. Also, you might want to configure 'mls qos trust device cisco-phone' so that DSCP is only trusted when a Cisco IP phone is plugged into the port.

In a Cisco IP phone deployment with a trust boundary on the phone, the phone typically does not trust the traffic coming from a connected PC. As a result, the phone does not mark the CoS (Class of Service) values for packets received from the PC. Instead, the access switch, which is typically configured to trust the phone's markings, will set or enforce the appropriate CoS values based on the network's QoS policies.