Router-on-a-Stick Was My First Clean Inter-VLAN Routing Lab
One router interface, an 802.1Q trunk and a few subinterfaces were enough to connect separate VLANs without hiding what was happening at Layer 2 and Layer 3.
802.1Q
Trunk Ports: The Part of VLANs I Initially Misunderstood
A trunk does not merge VLANs. It preserves multiple Layer 2 domains across one physical link by carrying VLAN identity with the frame.