Which of the following enables service providers to support multiple customers with the same VLAN ID over the same backbone?

VLAN Stacking, also known as Q-in-Q or IEEE 802.1ad, is the technology that enables this functionality. It works by adding a second VLAN tag, called the Service VLAN tag (S-VLAN or S-TAG), to the customer's original Ethernet frame, which already contains a Customer VLAN tag (C-VLAN or C-TAG). The service provider's backbone network then forwards the frame based solely on the outer S-VLAN tag, which is unique per customer. The inner C-VLAN tag is ignored by the provider's core but preserved, allowing different customers to use the same VLAN IDs without conflict. This effectively creates a tunnel for each customer's VLAN traffic across the shared provider network.

A. VLAN Trunking: Standard 802.1Q trunking carries multiple VLANs but cannot distinguish between identical VLAN IDs originating from different customers on the same backbone.
C. VLAN Tagging: This is the fundamental process of adding a single 802.1Q tag. It is the source of the problem (limited, overlapping VLAN space), not the solution.
D. VLAN Routing: This is a Layer 3 function for forwarding traffic between different VLANs. It does not address the Layer 2 transport of overlapping VLANs across a backbone.

1. Nokia 7750 SR OS Services Guide, Release 22.10.R1, Section: "QinQ (802.1ad)". The document states, "QinQ is a technique, also known as VLAN stacking, that is used by service providers to keep customer VLAN traffic segregated and to expand the number of VLANs supported in their network... The provider network transports customer traffic, keeping the customer’s VLANs separate, using a single provider-assigned VLAN."

2. IEEE Std 802.1ad-2005, "IEEE Standard for Local and metropolitan area networks - Virtual Bridged Local Area Networks - Amendment 4: Provider Bridges". Clause 6, "Provider Bridge architecture," describes the addition of a Service VLAN (S-VLAN) component and S-TAG to encapsulate Customer VLAN (C-VLAN) tagged frames, enabling the scaling and isolation required by service providers.

3. Stallings, W. (2016). Foundations of Modern Networking: SDN, NFV, and Cloud Computing. Pearson Education. Chapter 4.3, "IEEE 802.1 VLANs," discusses the limitations of the 12-bit VLAN ID and introduces IEEE 802.1ad (Q-in-Q) as the solution for service providers to handle traffic from multiple customers, each with their own set of VLAN IDs. This is a widely used textbook in university networking courses.