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

KB ID0001167

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.

Trunk Settings

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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23 Comments

  1. Thank you so much. You saved me a lot of stress.

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  2. Muchas gracias, fue de gran ayuda.

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  3. Thank you – I can’t believe I forgot this!
    It is indeed an old ISL throughback!

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