I've got a trunk link into which I've attached a pc. The pc's data is channeled via the native vlan, my question is how can I put dot1q encapsulation on the packets. I understand by native it remains untagged but is it still possible to tag it?
Just a small note, with a program like Ethereal aka Wireshark, your laptop does not need to support dot1Q encapsulation in order to record such frames.
What you could do:
1) Use a FE Hub and connect switches and laptop to it. You should be able to record any switch to switch traffic. Obviously anything above FE will not work.
2) Use a SPAN port to monitor traffic from a switch. This might or might not allow you to get the desired information depending on switch model and IOS version.
I have readily used option 1) to either understand protocols and features or to troubleshoot problems. The latter was done already quite some time back, as today most switch networks are on GE trunks and there are no hubs at those speeds.
Finally to answer your previous questions:
If the ping in your scenario above will be dot1Q tagged or not on the trunk, depends on the native VLAN for the trunk. The native VLAN by default will be 1 and thus: YES the data will be dot1Q tagged. But if you reconfigure the trunk to a native VLAN of 20, data will not be tagged.
Yes, you are right, this would create the same frames like on two laptops using dot1Q tagging.
Hope this helps!