@awebster
Thanks very much for the explanation. I think the situation is as follows:
in the original layer-2 ether-net specification there is no priority field
however there is a need for priority packets
in a later version of the layer-2 spec there is the 802.1Q tag which add
-3bit Priority code point (PCP) / COS
-1bit Drop eligible indicator (DEI) / CFI
-12bit VLAN identifier (VID) / vlan number
To transport un-tagged frames with a priority mechanism they defined a trick "vlan0". That trick adding an 802.1Q tag to the original layer-2 frame, allows the add of the PCP/COS and the DEI/CFI.
A managed (😊 ) switch receiving such an ^updated level2 frame^, can then process the frame with the correct priority.
Of cause the switch administrator can tell the swith that it should add "whatever vlantag / number" to that in coming untagged frame, where I assume that the DEI and PCP will be set accordingly in that vlan 802.1Q field.
And after transporting the frame to the other end of the network, another managed switch can output that frame to an untagged port. Doing the inverse trick changing the VID from whatever VID-value ("50") to 0.
One potential problem, assume we hand over that ^modern semi-un -tagged^ frame to an unmanged switch or an end device like a PC what will happen !?
Does the managed switch at the end have three options forwarding the package as:
vlan package with a real vlan number
as a vlan0 package perhaps not understandable for the attached equipment
or forwarding the package as a classical untagged package
Below a picture I took from https://en.wikipedia.org/wiki/IEEE_802.1Q
726cd7fa-108b-42c3-860d-a397f8f082a0-image.png
I hope I described it correct this way.
Louis