Dot1q - Tag Protocol Identifier - Why 16 bits?

[ndemers]

So this is more of a sanity check.  Im trying to find a reason why the TPI is 16 bits in dot1q.  the only thing that I can come up with is that it has to be the same length as ethertype (because it takes its place inside the frame) so that the switch will process it properly.

I know its 0x8100 but in a different world it might not be required to be 16 bits.  Why couldnt it be 8 bits?  To my knowledge other than the above mentioned guess is that it doesnt need to use that much overhead.  It seems pretty hard to find a definitive reason why its this way.

Any insight would be stellar.

[Reza Sharifi · Accepted solution]

Your understanding is correct.

Tag protocol identifier (TPID): a 16-bit field set to a value of 0x8100 in order to identify the frame as an IEEE 802.1Q-tagged frame. This field is located at the same position as the EtherType/length field in untagged frames, and is thus used to distinguish the frame from untagged frames.

http://en.wikipedia.org/wiki/IEEE_802.1Q

HTH

[ndemers]

I found this article which clarifies my Q.  It shows that an ethertype of 8100 denotes dot1q.  That was my main concern is if it was an ethertype and had to be 16 bits or if it was something else.

In reality the TPID is really just the ethertype field labeled differently.

http://blog.globalknowledge.com/technology/what-is-the-difference-between-ethernet-ii-and-ieee-802-3/

Thank you for the assistance