How is gateway on a stick inter-VLAN routing implemented?

How is gateway on a stick inter-VLAN routing implemented?

• A. Create a subinterface per VLAN on the router, assign an IP in each VLAN's subnet, configure 802.1Q encapsulation on each subinterface, and ensure the switch port toward the router is a trunk.
• B. Use a single interface for all VLANs combined.
• C. Configure the router as a Layer 3 switch with no subinterfaces.
• D. Run inter-VLAN routing on the switch.

Answer: (A)

Gateway on a stick relies on a single physical link between the switch and the router carrying traffic for multiple VLANs by tagging frames with 802.1Q. The router hosts a separate subinterface for each VLAN on that trunk, and each subinterface gets an IP address that serves as the default gateway for devices in its VLAN. The switch uplink to the router must be a trunk, so all VLAN tags traverse the link. Devices in each VLAN use the corresponding subinterface IP as their gateway, enabling inter-VLAN routing to occur on the router. This setup wouldn’t work if you tried to use one interface for all VLANs without tagging, because VLAN separation relies on tags to keep traffic distinct. If the router were configured as a Layer 3 switch with no subinterfaces, there would be no per-VLAN gateway on the router to route between VLANs over the trunk. And running inter-VLAN routing on the switch means the switch handles the routing itself, not the router via a stick, which is a different architecture from gateway on a stick.

Gateway on a stick relies on a single physical link between the switch and the router carrying traffic for multiple VLANs by tagging frames with 802.1Q. The router hosts a separate subinterface for each VLAN on that trunk, and each subinterface gets an IP address that serves as the default gateway for devices in its VLAN. The switch uplink to the router must be a trunk, so all VLAN tags traverse the link. Devices in each VLAN use the corresponding subinterface IP as their gateway, enabling inter-VLAN routing to occur on the router.

This setup wouldn’t work if you tried to use one interface for all VLANs without tagging, because VLAN separation relies on tags to keep traffic distinct. If the router were configured as a Layer 3 switch with no subinterfaces, there would be no per-VLAN gateway on the router to route between VLANs over the trunk. And running inter-VLAN routing on the switch means the switch handles the routing itself, not the router via a stick, which is a different architecture from gateway on a stick.