What does 802.1ad (Q-in-Q) add to VLAN tagging?

• A. An extra VLAN tag to carry multiple service attributes
• B. A second IP header
• C. A larger MTU
• D. A new EtherType

Answer: (A)

Q-in-Q is about stacking VLAN tags. It wraps an existing Ethernet frame with a second 802.1Q tag, so you can carry a customer’s VLAN inside a provider’s VLAN. The inner tag is the customer’s VLAN ID, and the outer tag is the service-provider VLAN ID. This creates a hierarchical, scalable tagging scheme that lets a service provider multiplex many customers over the same physical link while keeping traffic isolated and enabling provider-level service attributes (like QoS) to be applied. It doesn’t add a new IP header, doesn’t fundamentally change MTU beyond the small extra tag bytes, and it doesn’t introduce a new EtherType—the VLAN tagging method is simply applied twice.