Inter-VLAN Routing Using Router-on-a-Stick

Introduction

In a network, VLANs (Virtual Local Area Networks) are used to divide a single switch into multiple logical networks. But computers in different VLANs cannot communicate directly. That’s where Inter-VLAN routing comes in. It allows devices from different VLANs to communicate securely through a router.

One of the most common and cost-effective methods for Inter-VLAN communication is called Router-on-a-Stick.

---

What is Router-on-a-Stick?

Router-on-a-Stick is a setup where a single physical router interface is used to route traffic between multiple VLANs.
Instead of using multiple physical interfaces, the router uses subinterfaces one for each VLAN.
Each subinterface acts like a separate connection to a VLAN, identified by a VLAN ID.

This setup is called Router-on-a-Stick because a single connection (the stick) between the switch and the router handles all VLAN traffic.

---

How It Works (Step-by-Step)

1. Create VLANs on the switch (for example, VLAN 10 for Sales, VLAN 20 for HR, and VLAN 30 for IT).

2. Assign ports on the switch to each VLAN.

3. Configure a trunk link between the switch and the router so that multiple VLANs can share the same physical connection.

4. Set up subinterfaces on the router one for each VLAN.

   • Example:

     • Gig0/0.10 for VLAN 10

     • Gig0/0.20 for VLAN 20

     • Gig0/0.30 for VLAN 30

5. Assign IP addresses to each subinterface. These act as default gateways for devices in those VLANs.

6. Enable routing, so the router can pass traffic between VLANs.

Once configured, users in VLAN 10 can talk to VLAN 20 or VLAN 30 through the router without needing separate cables or interfaces.

---

Router-on-a-Stick Configuration Example

Router Configuration:

Router(config)# interface gig0/0
Router(config-if)# no shutdown

Router(config)# interface gig0/0.10
Router(config-subif)# encapsulation dot1Q 10
Router(config-subif)# ip address 192.168.10.1 255.255.255.0

Router(config)# interface gig0/0.20
Router(config-subif)# encapsulation dot1Q 20
Router(config-subif)# ip address 192.168.20.1 255.255.255.0

Router(config)# interface gig0/0.30
Router(config-subif)# encapsulation dot1Q 30
Router(config-subif)# ip address 192.168.30.1 255.255.255.0

Switch Configuration:

Switch(config)# vlan 10
Switch(config-vlan)# name Sales
Switch(config)# vlan 20
Switch(config-vlan)# name HR
Switch(config)# vlan 30
Switch(config-vlan)# name IT

Switch(config)# interface gig0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20,30

---

Advantages of Router-on-a-Stick

• Cost-effective  ] uses a single router interface.

• Easy to configure and manage.

• Ideal for small to medium-sized networks.

---

Limitations

• Not suitable for heavy traffic networks since all VLAN data passes through one link.

• Can become a bottleneck if too many VLANs or high data flow are involved.

---

Conclusion

Router-on-a-Stick is a simple and efficient way to enable communication between VLANs. It’s perfect for small networks or lab environments where cost and simplicity matter. Understanding this concept is also an essential step for anyone preparing for CCNA or networking certifications.