
switch(config-if)#switchport mode trunk Command rejected: An interface whose trunk encapsulation is “Auto” can not be configured to “trunk” mode.
This is a result of a missing encapsulation command type (That is 802.1Q on that port)
– Enter the interface configuration mode and enter the following command below
switch(config-if)#switchport trunk encapsulation dot1q
Before proceeding with the rest configuration
– The port can now be set as a trunk port without any issue.
switch(config-if)switchport mode trunk