Command rejected: An interface whose trunk encapsulation is “Auto” can not be configured to “trunk” mode.

Working on setting up a trunk interface on one of my switches, came across an issue that I just resolved. Thought I would share:

When I was first attempting to create a trunk interface I got the following error:

Example(config-if)#switchport mode trunk
Command rejected: An interface whose trunk encapsulation is “Auto” can not be configured to “trunk” mode.

Searched for a way to turn off the “auto” config on the interface but could not find it. After some research, found that I need to first set the encapsulation on the interface which will then give me the the ability to set this interface as “trunk.”

Example(config-if)#switchport trunk encapsulation dot1q

This is an intrusive config, the interface will drop but then come back up. Once the interface is set with the encapsulation type we can set the interface as trunk.

Example(config-if)#switchport mode trunk
Example#show run int eth 1/0
Building configuration…
Current configuration : 103 bytes
!
interface Ethernet1/0
switchport trunk encapsulation dot1q
switchport mode trunk
duplex auto
end
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