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Router on a Stick (ROAS): A Practical Guide to Inter-VLAN Routing

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In modern networks, Virtual Local Area Networks (VLANs) are commonly used to segment traffic for better organization, security, and efficiency. However, devices in different VLANs cannot communicate with each other without a routing device. One of the low cost, effective and straightforward technique that can enable inter-VLAN communication is known as Router on a Stick (ROAS).

What is Router on a Stick (ROAS)?

Router on a Stick is a network configuration that enables inter-VLAN routing using a single physical interface on a router. It allows multiple VLANs on a switch to communicate through one trunk link between the switch and the router. This setup is commonly used in small to medium-sized networks due to its simplicity and cost-effectiveness.

How Router on a Stick (ROAS) Works?

  1. Single Physical Link
    • One Ethernet cable connects the router and the switch.
    • This link is configured as a trunk port to carry traffic from multiple VLANs.
  2. VLAN Tagging (802.1Q)
    • The switch tags each frame with a VLAN ID using the 802.1Q protocol.
    • The router reads these tags to direct traffic appropriately.
  3. Router Sub-interfaces
    • The physical router interface is divided into virtual sub-interfaces.
    • Each sub-interface:
      • Is assigned to a specific VLAN.
      • Has its own IP address (acts as the default gateway for that VLAN).
      • Handles traffic for that VLAN using VLAN tagging.
  4. Inter-VLAN Routing
    • The router routes traffic between VLANs internally using its routing table.
    • Devices in different VLANs communicate via the router’s sub-interfaces.

Benefits of Router on a Stick (ROAS)

  1. Cost-Effective
    • Eliminates the need for expensive Layer 3 switches.
    • Ideal for budget-conscious or smaller network deployments.
  2. Simplified Setup
    • Easier to configure in environments with limited routing infrastructure.
    • Uses familiar commands and interface settings for most network admins.
  3. Scalable for Moderate Networks
    • Handles inter-VLAN routing for networks with moderate traffic.
    • Suitable for labs, branch offices, and SMBs.

Limitation of Router on a Stick (ROAS)

  1. Limited Bandwidth
    • All inter-VLAN traffic shares one physical link, potentially causing a bottleneck in high-traffic environments.
    • Single Point of Failure
    • If the router or trunk link fails, all VLAN communication stops.
  2. Requires Technical Understanding
    • While simple in design, it demands a working knowledge of:
      • VLANs
      • Sub-interface configuration
      • Trunking and routing principles
  3. Not Suitable for Large Networks
    • High-performance or enterprise networks should use Layer 3 switches instead for better speed, redundancy, and scalability.

When to Use and Not Use Router on a Stick (ROAS)?

Best suited for:

Avoid in:

Conclusion

Router on a Stick (ROAS) is a practical, low-cost method for inter-VLAN communication using a single router interface and a trunk link. While it’s not suitable for high-traffic or enterprise environments, it’s an effective solution for smaller networks that need simple and reliable VLAN routing.

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