Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.
dot1q Interface uses only 802.1q trunking encapsulation when trunking
Switch3560(config-if)#switchport trunk encapsulation dot1q //先要输入封装格式
Switch3560(config-if)#switchport mode trunk //可以顺利的输入这个命令了