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

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KB ID0001167Dtd19/03/16

Problem

If you try and change a ports status, to make it a trunk port, you may see this error;

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

Solution

I don't know if this is a throwback to when we had ISL trunking and 802.1q, but you need to specify the encapsulation before you can specify a trunk.

Petes-Switch(config-if)#switchport mode trunk
Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.
Petes-Switch(config-if)#switchport trunk encapsulation dot1q
Petes-Switch(config-if)#switchport mode trunk
Petes-Switch(config-if)#

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Author: PeteLong

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