单项选择题 Three switches are connected to one anther via trunk ports.Assuming the default switch configuration,which switch is elected as the root bridge for the spanning-tree instance of vlan 1?（）

单项选择题 What is the purpose of the cisco Vlan Trunking protocol?（）

单项选择题 Which cisco ios command can help to determine the timing of various debug events,relative to each other,when you are debugging a complicated router issue?（）