Not able to change auto to trunk mode on switch

Unanswered Question
May 24th, 2006

SW-B-TOP(config-if)#switchport mode trunk

When trying to execute the above command am getting the following error.

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

I have this problem too.
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Overall Rating: 3.8 (9 ratings)
glen.grant Wed, 05/24/2006 - 15:30

You have to specifically set the encapsulation type first "switchport encapsulation dot1q" then try youir command...

aksher@cisco Wed, 05/24/2006 - 15:47

it's not accepting.

SW-B-TOP(config-if)#switchport encapsulation dot1q


% Invalid input detected at '^' marker

the model is CGESM.

glen.grant Wed, 05/24/2006 - 16:32

You need to specifically set the encapsulation first , "switchport encapsulation dot1q" then issue your trunk command...

ankurbhasin Wed, 05/24/2006 - 22:00

Hi All,

The command which he need to enter first is

"switchport trunk encapsulation dot1q " and then he can confgigure the interface in trunk mode.

So try this command

switch(config)#interface fa0/1

switch(config-if)#switchport trunk encapsulation dot1q

switch(config-if)#switchport mode trunk

and you will be good to go.

HTH, if yes please rate the post.


aksher@cisco Thu, 05/25/2006 - 10:13

Thanks Ankur

Should we need to disable the concerned vlan of the port before entering the configuration.

ankurbhasin Fri, 05/26/2006 - 00:31

Hi Friend,

Thats not required because if you put the config which I posted in my previous mail it will take care of the issue.

HTH, if yes please rate the post.


FatimaBeyelhadj Wed, 10/02/2013 - 09:28

This worked on one of the switches and not the other?! 

What do you think could be the cause besides lab technical issues?

Serge Wed, 10/09/2013 - 05:39

Could you run a " sh int gig #/# capabilities"

look under Trunk encap. type:

and Trunk mode:

it will give you capabilities information of the port

jhotles@unityte... Mon, 10/21/2013 - 07:01

The command is "switchport TRUNK encapsulation dot1q".  This will then allow the command "Switchport MODE TRUNK".

This is the core to the configuration - You NEED this command to run a standard trunk configuration otherwise it WILL NOT work...

zul qarnain Fri, 06/13/2014 - 17:14

Hi Ankur,

I have question why for switch multilayer is different from switch not multilayer if we need configure trunk on port?



Sudeb Das Fri, 07/17/2015 - 21:45

Akser I believe u r trying the command on u r gns3 switch. It will reject the command. Check the ios version of u r switch in gns3. 

In real world the cisco switches will accept the command without an error. 


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