Implementing the Router-on-a-Stick Solution

Router-on-a-stick is a router configuration that allows multiple VLANs to communicate through a single physical router interface. This is achieved by configuring a router to use a single physical interface to connect to a switch, which is connected to multiple VLANs.

The switch port connected to the router is configured as a trunk port to carry VLAN traffic to and from the router for multiple VLANs. The router is then configured with sub-interfaces, which are logical interfaces operating the same as the physical interfaces on the router.

Each sub-interface is associated with a specific VLAN and is configured with a unique IP address within that VLAN. This allows the router to route traffic between the VLANs on the switch while ensuring each has its unique network address.

As you can see in the image above, two subinterfaces are configured on the router, each connecting to a different VLAN. Subinterface 10 (Fa0/1.10) connects to VLAN 10 and belongs to the same network 192.168.10.0/24 as any device in VLAN 10. The 192.168.10.1 IP address defined on the subinterface is used as a default gateway for devices in VLAN 10, including PC1. The same applies to the other subinterface 20 and VLAN 20.

This solution is commonly used in environments where a large number of VLANs need to communicate with each other but where there are only a limited number of physical router interfaces available or switches with no routing capabilities are used.

Although this solution is considered old-school, it has been proven effective and provides an efficient and cost-effective way to implement inter-VLAN routing without additional physical interfaces or Layer 3 switches.

Disadvantages of the Router-on-a-Stick Solution

While this approach can be cost-effective and efficient in certain scenarios, it also has some disadvantages:

• Single point of failure: If the router interface that connects to the switch fails, all VLANs connected to the switch become unreachable, leading to network downtime.
• Limited bandwidth: Since all traffic from the VLANs is routed through a single interface, there can be a bottleneck and limited bandwidth, leading to slower network speeds.
• Complex configuration: Configuring a router-on-a-stick solution requires a more complex configuration, especially if there are many VLANs.
• Limited security: A router-on-a-stick solution provides limited security since all traffic from the VLANs shares the same physical interface. This makes it easier for attackers to access all VLANs if they breach the network's perimeter.