Priority Tagging and VLAN Tags

Originally, the standards effort for implementing frame priority (P802.1p) was completely separate from the effort for implementing VLANs (P802.1Q). It was thought that priority could be a simpler feature to add than VLANs, and one could build low-cost "priority capable" switches without the complexity of a full VLAN implementation.

In the end, we combined the projects, and the priority mechanism (priority tags) was essentially the same as the VLAN mechanism (VLAN tags). Thus, it really made no sense to build a "priority-capable" switch that was not also VLAN-capable; there was no cost benefit to do so.

Given that situation, there is rarely a need to use "pure" priority tags, i.e., to assign priority to a frame without also assigning the frame to a VLAN. It is still possible to do so; the VLAN ID 0x00 is reserved for that purpose. However, the same feature (frame priority) is also achieved when assigning a non-zero VLAN ID. Thus, if there is a default VLAN in a given switch design, it probably makes more sense to assign the frame to the default VLAN (and insert the desired priority value), rather than assign a priority through the use of priority tags.

I hope this helps. And thanks for buying the book!

I think I get it now, let me play it back to make sure: to implement priority rather than use a VLAN identifier of 0 if the station does not belong to a VLAN, devices would simply drop the frame into a default VLAN and treat the frame as a VLAN tagged frame with priority. Right?

That clicks with my brain a bit more for some reason.

Thanks! (for the answer and for writing the book)

Correct.

This was very logical and those of us on the deployment end appreciate only having one tag to deal with, even if it has two purposes :)

Right. However, hosts are typically unaware of the VLAN they're assigned to (or are prohibited from using VLAN tags for security reasons), so a VLAN ID of 0x0 is needed for the link between the switch and the host if priority tags are being used.

That said, I can't recall seeing anyone use priority tags with hosts that weren't also configured to (and allowed to) use VLAN tags. The usual case is a VoIP phone that does VLAN and priority tagging to the switch but passes through PC traffic (from a daisy-chain port) untagged. Now that I think about it, I can't recall having seen priority frames used for _anything_ else.

In addition enduser priority assignment is stillborn in the first place. It is about as reasonable as letting kids choose between icecream and supper. Doing it by tags eases insecure implementations as an enduser port has to be characterized as "admidAll" instead of admitOnlyUnTagged

There are layer-3 switches that priorize frames containing routing protocol information.