A.at the PC
B.at the access switch
C.at the IP phone
D.at the distribution switch

A.Trust boundaries are configured in the core of the network to provide the most efficient  forwarding based upon QoS markings，
B.Setting trust boundaries at the edge of the network allows intermediate hop devices to  administer QoS policies without detailed packet identification.
C.Trust boundaries are used to determine which QoS mechanism will be applied to the traffic.  
D.At the trust boundaries， the untrusted traffic will be marked with a new QoS value appropriate  for the policy in place at the point where the traffic entered the campus network.  
E.Trust boundaries define the firewall rules for QoS admission into a network.