In this article, we shall discuss the fix for “Error – Command rejected: An interface whose trunk encapsulation is “Auto” can not be configured to “trunk” mode”. The error message is prompted when you attempt to configure an interface to trunk mode. While its encapsulation is set to “Auto.” This happens when the switch cannot determine the encapsulation method (IEEE 802.1Q or ISL) for the trunk link. Please see How to disable auto login in Windows, and how to Disable or Enable Auto log on in Windows.

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Reason for the error

As you can see below, the error is prompted. This is because, you will need to explicitly set the trunk encapsulation before configuring the interface to trunk mode.

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). Follow the steps below to fix this “Command Rejected” error message.

Set the Encapsulation Type

Before setting the interface to trunk mode, you need to specify the encapsulation type. Enter the interface configuration mode and enter the following command below

switch(config-if)#switchport trunk encapsulation dot1q

Before proceeding with the rest configuration. You can now set the port as a trunk without any issue.

Configure the Interface as Trunk

switch(config-if)switchport mode trunk

Exit the interface configuration mode and save the changes.

exit
write memory           # Save the configuration

For other guides, see: How to clear Cache and Manually Update Microsoft Defender, How to fix: This network connection does not exist and Reveal the WiFi password of the Network you are connected to in Windows.

I hope you found this blog post on fixing this “Error – Command rejected: An interface whose trunk encapsulation is “Auto” can not be configured to “trunk” mode” helpful. If you have any questions, please let me know in the comment session.