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

By | August 1, 2015

Did you get the following output trying to set a port in trunk mode on a Cisco IOS powered switch?

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

This is because you are missing the encapsulation type (likely to be 802.1Q) on that port.

Using the configuration mode on that interface, enter the following command :

Then you may set the port as trunk :