If a trust boundary is placed on a Cisco IP phone, what occurs with the packets that come to the phone from a PC?

The phone will not set PC data traffic to CoS 3 or 5; instead, it resets untrusted traffic from the PC to a CoS value of 0 to remove any potentially fraudulent markings.

If the phone is established as the trust boundary, it actively marks its own voice packets with appropriate CoS values and resets CoS values for PC data packets, rather than leaving all marking solely to the access switch.

A Cisco IP phone configured as a trust boundary ensures that any data traffic originating from a PC connected to its auxiliary port has its CoS value reset to 0, preventing untrusted applications from influencing QoS. Concurrently, the phone correctly marks its own voice signaling with CoS 3 and voice media with CoS 5, allowing the upstream switch to prioritize these packets based on trusted markings.

QoS marking for endpoint-originated traffic should occur as close to the source as possible, typically at the access layer where the trust boundary is defined on the phone, not primarily at the distribution layer for initial marking.