Hi Stefanio,
I understand this can be confusing. When you read about VLAN hopping, they usually talk about interfaces in access mode that accept frames with two VLAN tags. The first tag is ignored, the second one allows you to jump from one VLAN to another.
The 3560 switch that I used didn’t like this at all…it does not accept a frame that is tagged if the interface is configured in access mode. I didn’t test it but older switches (or IOS images), or other vendors switches, might accept a double-tagged frame on an access mode interface, making VLAN hopping possible.
In my example, I had to set Fa0/1 on SW1 to trunk mode for it to accept double VLAN tags. On modern IOS switches, VLAN hopping on access mode interfaces is no longer an issue.