What is the correct term for the inner VLAN tag in a Q-in-Q tag?

In the context of Q-in-Q tagging, the inner VLAN tag is referred to as the C-VLAN or Customer VLAN. Q-in-Q is a technique used to encapsulate Ethernet frames with multiple VLAN tags, facilitating the handling of traffic from multiple customers over a shared network infrastructure.

The C-VLAN specifically identifies the traffic originating from a single customer, allowing service providers to segregate customer traffic while still utilizing a shared backbone. This tagging method enhances bandwidth management and prevents issues arising from VLAN ID limitations in the service provider's network.

By contrast, the S-VLAN, or Service VLAN, is the outer tag used by the provider to manage and forward the encapsulated customer traffic efficiently. P-VLAN and D-VLAN do not pertain to the Q-in-Q tagging scheme in the same direct way and are less commonly referenced terms within this specific context. Understanding these terms is essential for working with VLANs in Carrier Ethernet environments and for ensuring the correct flow of data between diverse customer networks.