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I read about tagged ethernet frames, which include the extra 802.1Q field, and understand the concept and operation.

One thing I wonder about is whether there is a way for a node receiving an Ethernet frame to distinguish between tagged frames and non-tagged frames.

I read in answers about Trunk ports where it's possible to have on untagged vlan on each port (e.g. https://networkengineering.stackexchange.com/a/10565/19642), so how does the switch know that the incoming frame is untagged? Is it just assumed based on the fields of the 802.1Q header not having sensible values?

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The switch / NIC looks for the tag in each frame and since it's got it's special TPID value 0x8100 (where otherwise the Ethertype would be) the following two bytes are interpreted as PCP and VLAN ID values, and then the true Ethertype and the rest of the frame follows.

An untagged frame coming in from a VLAN trunk port is simply interpreted as belonging to the untagged VLAN.

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  • Thanks. So basically the tagged frames can be recognised by looking for a specific value in what would be The Ethertype field.
    – Capt. Crunch
    Jun 18, 2017 at 7:51

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