Switch – Can a non-VLAN-aware switch pass 802.1q tagged packets

The normal use of VLANs is to separate subnets (e.g. 192.168.0.0/24, 10.20.0.0/16, etc) without using separate switches for each subnet.

The point of the PVID is to have the switch tag incoming untagged packets, say from a workstation or server. Normal NIC configurations do not tag packets for most devices.

If you were to connect a device capable of tagging its own packets, then it could talk along any VLANs the port has been given membership to. This is handy when you're connecting switches together, connecting to a router that understands VLANS, or using a server that needs to be able to connect to several subnets (virtualization, especially).

I have actually seen this on a cheapo-switch. Someone had connected a switch between a trunk port which had a couple vlans. The frames were forwarded with the vlan tagging intact. The other ports on that switch where able to use the un-tagged vlan.

A switch only needs the source/destination mac to decide which ports to forward the frames to, so this isn't too surprising, a tagged frame still has the source and destination macs, in the same location in the frame header.

Keep in mind that Ethernet actually supports many different frame-types on the same wire. It was designed to be pretty flexible about what it can do.