多项选择题 You want to enhance the security within the LAN and prevent VLAN hopping.  What two steps can be taken to help prevent this？（）

单项选择题 The network administrator has enabled BPDU guard on the LAN.  Why is an effective way to prevent an unauthorized rogue switch from altering the spanning-tree topology of this network？ （）

单项选择题 Refer to the exhibit.  What is the sequence of events when a VoIP call is placed from IP phone P1 to IP phone P2？ （）