问题：

[多选] Which two statements are true about trust boundaries？（）

A . Classifying and marking traffic should be done as close to the traffic source as possible. 
B . Classifying and marking traffic should be done at the distribution layer.
C . Traffic is classified and marked as it travels through the network.
D . If untrusted traffic enters a switch， it can be marked with a new QoS value appropriate for the policy in place.
E . The trust boundary moves depending on the type of traffic entering the network.