IEEE Q defines VLAN tagging. It lets one physical network carry multiple isolated virtual networks.
How it works:
A -byte tag inserts into the Ethernet frame containing:
- TPID: x indicates Q
- PCP ( bits): Priority for QoS
- VID ( bits): VLAN ID from to
Why it matters:
Without Q, VLANs would not work across different vendors. A Cisco trunk could not connect to an Arista switch.
ad adds Q-in-Q double tagging for service providers.