QinQ and duplicate VIDs

Duplicate VID's for c-tagged and s-tagged VLANs (for example, C-VID=100; S-VID=100) are allowed in certain cases. Customer-network ports are essentially S-VLAN ports: they simply read the C-tags in the customer frame to insert them into the appropriate untagged S-VLAN for that port. Once this double-tagging occurs, frames are forwarded based on the S-VLAN tag only, while the C-VLAN tag remains shielded during data transmission.
QinQ and duplicate VIDs: examples of allowed configurations