Refer to the exhibit.

What is the effect on the trust boundary of configuring the command mls
qos trust cos on the switch port that is connected to the IP phone?

A.
Effectively the trust boundary has been moved to the IP phone.

B.
RTP will be used to negotiate a CoS value based upon bandwidth utilization on the link.

C.
The switch is rewriting packets it receives from the IP phone and determining the CoS
value.

D.
The switch will no longer tag incoming voice packets and will trust the distribution layer
switch to set the CoS.

E.
The host is now establishing the CoS value and has effectively become the trust
boundary.

Explanation:
Configuring Trusted Boundary:
In a typical network, you connect a Cisco IP Phone to a switch port. 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 CoS 3-bit field, which determines the priority
of the packet. For most Cisco IP Phone configurations, the traffic sent from the telephone to
the switch is 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

can configure the switch port to which the telephone is connected to trust the CoS labels of
all traffic received on that port.
Reference:
http://www.cisco.com/en/US/docs/switches/lan/catalyst2940/software/release/12.1_19_ea1/c
onfiguration/guide/swqos.html