Trunking

[52j22304]

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

I got this message while I'm configring etherchannel, in specific it was in " interface port-channel 2" 

[M02@rt37]

Hello @52j22304 

Please try enter the command "switch(config-if)#switchport trunk encapsulation dot1q" first.

Then try to configurre the interface as a trunk again.

[balaji.bandi]

Agreed with M02@rt37 - the command need to apply on the Physical interface or interface port-channel.

[MHM Cisco World]

Port channel need all port member hardcoded it trunk encap and trunk/access mode 

I.e. you can use DTP and auto encap for port member

Why?

image one port is access and other is trunk because we use DTP

Or 

One port encap ISL abd other dot1q

So it mandatory to hardcoded these settings before port is bundle to PO

MHM