CompTIA Network+ (N10-009)Network ImplementationMedium
A network administrator wants a single Layer 3 switch to route traffic between VLAN 10, VLAN 20, and VLAN 30 using only one physical interface connected to a Layer 2 switch trunk. Which configuration approach should the administrator use?
- AEnable port security on the single trunk interface to separate VLAN traffic
- BConfigure the single interface as an access port and assign it to VLAN 10 only
- CConfigure a router-on-a-stick using subinterfaces, each assigned to a VLAN and given an IP address
- DConfigure three separate physical interfaces, one per VLAN, all in access mode
Show answer & explanationAnswer & explanation
Correct answer: C. Configure a router-on-a-stick using subinterfaces, each assigned to a VLAN and given an IP address
Router-on-a-stick uses a single physical trunk interface divided into logical subinterfaces (e.g., Gi0/0.10, Gi0/0.20), each tagged for a specific VLAN with 802.1Q encapsulation and assigned an IP address to act as that VLAN's gateway, enabling inter-VLAN routing over one physical link.
Why the other options are wrong
- A. Port security controls MAC addresses, not VLAN routing.
- B. An access port for one VLAN cannot carry or route traffic for other VLANs.
- D. This requires three physical interfaces, defeating the single-interface requirement.
Router-on-a-Stick
A configuration where a single physical router or switch interface is divided into 802.1Q-tagged subinterfaces to provide inter-VLAN routing.
- Requires a trunk link to the switch
- Each subinterface gets its own IP and VLAN tag
- Alternative to using a Layer 3 switch with SVIs
Memory trick: One stick, many branches — one wire, many VLANs.