A. Default Mode (without any configuration, the PMP450 bridge is transparent, it passes all traffic, including Q-in-Q)
B. You can configure the PMP450 SM to add a S-tag to the traffic
C. To configure Q-in-Q,
1> enable VLAN on the AP
2> On the SM, choose VLAN Port type Q-in-Q in VLAN configuration
3> On the SM Choose "Enable" for Accept QinQ Frames
4> On the SM, configure default Port VID - this will be your C-tag
5> On the SM, configure Provide VID - this will be your S-tag, don't use "1", if you use "1" nothing will happen.
Now, untagged traffic comming from the LAN port will be tagged with S-tag/C-tag when it comes out of the AP.