思科交换机没有 Trunk 封装选项

Switch(config-if)#switchport trunk ?
  allowed  Set allowed VLAN characteristics when interface is in trunking mode
  native   Set trunking native characteristics when interface is in trunking
           mode
Switch(config-if)#switchport trunk encapsulation dotlq
                                   ^
% Invalid input detected at '^' marker.

原因是新的思科交换机设备只支持802.1q的封装格式，故没有封装选项，直接 switchport mode trunk 就可以了。

参考：https://community.cisco.com/t5/other-network-architecture/2960-will-not-allow-quot-switchport-trunk-encapsulation-dot1q/td-p/2439012