Today I will share the brief of understanding regarding detection of tagged frames on UNI side which I have gained after working on this topic.
In earlier ciena release(s), VLAN tagged frames which are received on the UNI side were checked to see if their VLAN TPID matches with by-default system wide TPID of 0x8100. If received frame VLAN's TPID matches 0x8100 then that frame is considered as Single tagged frame.
If received frame has VLAN TPID other than 0x8100 (for e.g., 0x88a8 or 0x9100) then those frames were considered to be untagged frames. Those frames will get processed in the way you have defined semantics to handle untagged frame received on the UNI side.
Even if on the UNI side is receiving two VLAN tags then outer VLAN TPID is matched with system wide default TPID setting 0x8100 and if TPID matches then frame will still be considered as single tagged frame irrespective of how many VLAN tags were present after the outer VLAN tag on the frame received on the UNI.
In the new releases, ciena devices are coming up with the customization of system wide TPID value to change to any of the values from the list (0x8100, 0x9100 or 0x88a8). After you change the system wide TPID value to non-default value then frame received on the UNI with VLAN TPID matching to new system wide TPID setting will be considered as single tagged frame and non-matched frames will be considered as untagged frame.
For e.g. frame recieved on UNI has VLAN TPID= 0x88a8 and system wide TPID setting is also changed to 0x88a8. Then UNI frames with TPID matching to 0x88a8 will be considered as single tagged frame and other TPID like 0x8100 and 0x9100 on the UNI side will be considered as untagged frame.
Hope it helps.