Router on a Stick Configuration: Complete Guide

Inter VLAN connectivity also known as "router on a stick" is used to connect different VLANs. In traditional network setups, VLANs are isolated from each other and cannot directly communicate. However, by implementing Inter VLAN connectivity, traffic can be routed between VLANs using a single physical interface on a router.

The concept of the router on a stick involves connecting a router to a switch and configuring sub-interfaces on the router's interface. The router's sub-interfaces are configured in different VLANs with each VLAN having separate network subnet IP addresses.

The router's sub-interface IP address becomes the gateway for that particular VLAN subnet hence allowing traffic between VLANs. The switch interface connecting to the router is configured as a trunk link and those VLANs should be allowed through this trunk link.

In this guide, we will learn about router on a stick and how to configure router on a stick in Cisco devices. We will be doing a practical, and mention step-by-step process for router on a stick configuration. This guide is useful for people pursuing any Cisco certification training.

What is Router on a Stick (ROAS)?

Router on a Stick (ROAS) is a network configuration that enables inter-VLAN connectivity using a single physical router interface. This method is essential for organizations with multiple VLANs, allowing efficient communication between them.

By utilizing subinterfaces, routers can manage traffic effectively, reducing the need for multiple physical connections. As businesses increasingly adopt VLANs for improved security and performance, ROAS becomes crucial in ensuring seamless data flow across different network segments.

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Topology Diagram

How to Configure Router on a Stick

Let's see the steps to configure inter-Vlan connectivity in Cisco devices. You can practice these steps in our virtual lab.

Tasks

*Note enable password is CISCO

● Login to SW03 and confirm this switch has VLAN 3 and VLAN 4 configured. VLAN 3 should be assigned to interface E1/0 and VLAN 4 should be assigned to interface 0/0.

 ✓ If these VLANs don't exist or aren't configured on the access ports specified above, configure it.

● Configure interface E0/3 on SW03 to operate as an 802.1q VLAN Trunk.

● Configure router R03 as a router-on-a-stick such that it will be able to route for both VLAN 3 and VLAN 4

 ✓ Assign the first available host address for the relevant subnets to R03's sub-interfaces.

● Configure IP address 170.50.0.162 255.255.255.224 on PC03 interface E0 with gateway IP as 170.50.0.161, and IP address 170.50.0.130 255.255.255.224 on PC interface E0 with gateway as 170.50.0.129.

Configuration and Verification 

Here on SW03, VLAN 3 and VLAN 4 are already configured in previous tasks, and both are assigned to interfaces connected to PC and PC03. 

SW03:

Configure interface E0/3 on SW03 as 802.1q trunk which is connected to router R03

Create two sub-interfaces on R03 for VLAN 3 and VLAN 4, and assign the first IP address in the subnet of segments E and F respectively. 

R03:

Now configure IP addresses on PC and PC03 as provided in the task. Note: These are tiny Linux machines and have a simple CLI to configure IP address parameters. 

Note: It is recommended to set the hostname for these PCs as these have having default name VPC. 

PC:

PC03:

Verify the connectivity from PC to PC03, these PCs are in different VLANs, and they can ping each other meaning that inter-VLAN connectivity is successful. 

As you can see in the above output PC can ping its gateway and can also ping PC03 which is in the other VLAN.

Note: disable ip cef using the command "no ip cef" in configuration mode on SW03 if required. Use CTL+c to stop the ping process on the PC. 

Router on a Stick - Summing Up

In conclusion, Router on a Stick (ROAS) is an effective method for enabling inter-VLAN connectivity using a single router interface. Cisco devices facilitate this configuration by creating subinterfaces for each VLAN.

This setup optimizes bandwidth usage and simplifies network management, allowing efficient communication between different VLANs while minimizing hardware requirements. In this article, we have covered the benefits, needs, and configuration of a Router on a stick (ROAM).

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