I’ve noticed that a lot of people seem to get confused with what exactly dot1q is doing most of the time. It’s actually incredibly simply. Tagging traffic, or Trunking in Cisco-talk, is a very straightforward process. I will not be discussing ISL here as not only do I not use it, but Cisco is phasing it out on their stuff anyway. The dot1q tag is simply inserted into the layer2 header when a packet leaves a switchport over a trunk. If a frame leaves a switchport that is not a trunk, there is no dot1a tag inserted into it, regardless of what vlan the frame came from or is going to.