routeronastick

Router on a stick is a network design in which a single router interface connects to a switch via a trunk carrying multiple VLANs. The router then performs inter-VLAN routing by using subinterfaces, with each subinterface handling a separate VLAN and providing a distinct IP gateway for hosts in that VLAN.

Implementation typically involves creating a subinterface for each VLAN on the router (for example, interface Gig0/1.10 with encapsulation dot1q 10 and an IP address for the VLAN). The switch port that connects to the router is configured as a trunk, carrying the relevant VLANs, while end devices connect to access ports assigned to their respective VLANs. Hosts in each VLAN use the router’s subinterface IP as their default gateway.

The design requires 802.1Q compatible devices and proper trunk configuration on both sides. The native VLAN traffic is usually carried untagged on the trunk, so care is needed to align VLAN assignment and security policies. Network administrators may apply access control lists or firewall rules on the router to control inter-VLAN traffic.

Advantages of router on a stick include simplicity and lower hardware costs, as it avoids needing a separate Layer 3 switch for small networks. It’s well suited for small offices, branch offices, or lab environments where VLANs are modest in number and throughput needs are modest.

Disadvantages include limited scalability and potential performance bottlenecks on the router, since inter-VLAN routing is centralized on a single device. It can also introduce a single point of failure and more complex troubleshooting as VLAN counts grow. Alternatives include dedicated Layer 3 switches or multiple physical router interfaces, depending on performance and scalability requirements.