Just a basic configuration would be.
switchport trunk encapsulation dot1q
switchport mode trunk
Some switches only support one encapsulation type on a trunk. In that case, there is no command "switchport trunk encapsulation dot1q". Some older Cisco stuff only done the proprietary ISL trunking. Now most switches do the standard dot1q or both methods.
Paul's answer is right on the money.
If you are using Packet Tracer, it may not support the command switchport trunk encapsulation dot1q, but will still use it as a default. You will still be good to go if you use the switchport mode trunk command.
Just adding a little note here, if you wish to configure an ISL trunk, use 'switchport trunk encapsulation isl' instead.
Also, if you want to create a trunk link between a switch and a router, you only need to configure trunk on the switch MANUALLY because the router doesn't know what is trunking....
I think trunking has to be setup on the router port that connects to the trunking port of the switch.
On the Cisco ASR 1002 router you would have to create a sub interface and configure the dot.q trunking on it. then configure the ip addresses of the sub-interface for each vlan.
Have a great day!
CCNA, MCSE, MCDBA