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

This error is returned by Cisco IOS, when the trunking encapsulation is not set on one side of the switch and the other hand switch is configured properly. One of the case may be, when trunking between the port of a switch on one side and port of Switch-type interface on the router on other side. As you know that Dynamic Trunking Protocol, DTP, is not run by the Router so the Trunking can not be negotiated at this situation. To mitigate the error at this situation do the following steps;


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


Go to the desired Interface where you are getting this error
networkpcworld(config)#int fa0/0/1


Enable trunk ecnapsulation manually 
networkpcworld(config-if)#switchport trunk encapsulation dot1q
Note:- After setting encapsulation, most probably the switch interface would be converted to trunk by switchport mode trunk command. If not then enter the below command, at the same interface. 


Set the port to dynamic Desirable
networkpcworld(config-if)#switchport mode dynamic desirablenetworkpcworld(config-if)#switchport mode trunk

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