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Table of Contents
VLANs are used to separate different subnetworks in a large network. When a network is divided into VLANs, each VLAN becomes an independent logical network. Devices located in different VLANs cannot communicate without a routing process. This limitation also applies to VLAN environments. Since each VLAN acts as a separate broadcast domain, communication between VLANs is not possible by default. To enable this communication, routing between VLANs is required. This process is called Inter VLAN Routing. And one of the ways of this routing is using Router on Stick topology.
Inter VLAN Routing is the Routing that is done between different VLANs. It can be implemented in two different ways:
We will explain both methods in detail.
Router-on-a-Stick topology is a traditional method of Inter VLAN Routing. In this method, a Layer 3 device (router or multilayer switch) is used and connected to a Layer 2 switch through a single physical interface. This physical interface is divided into multiple subinterfaces, and each subinterface is assigned to a specific VLAN. Each subinterface acts as the default gateway for its VLAN.
On the switch side, the port connected to the router must be configured as a trunk port to carry VLAN-tagged traffic. In other wordws, the traffic from multiple VLANs is carried over a single trunk link between the switch and the router. The router then forwards traffic between VLANs using its subinterfaces.
Without a trunk link, separate physical links would be required for each VLAN, which is not efficient or scalable.
Inter VLAN Routing With Router-on-Stick Topology
Inter VLAN Routing with Router on Stick topology can be done for the VLANs on Layer 2 or Layer 3 switches. But here, there is a single Layer 3 device, a router or a multilayer switch is required at the top.
We have learned what is Inter VLAN Routing and what is Router-on-Stick topology. How does Router-on-Stick Work? What are the operation steps? Consider the below example topology where a PC in VLAN 2 communicates with a PC in VLAN 4.
Inter VLAN Routing – Router-on-Stick Operation
The Router-on Stick Operation works as follows:
Now, let’s talk about the configuration steps of Router on Stick Topology. The configuration of Router-on-a-Stick consists of four main steps:
In the first step, required VLANs are created on the switch and access ports are assigned to the appropriate VLANs.
In the second step, the switch port connected to the router is configured as a trunk port and allowed VLANs are defined.
On the router, subinterfaces are created under the physical router interface. Each subinterface is assigned an IP address and configured with 802.1Q encapsulation.
Connectivity is tested using ping and configuration is verified using show commands.
Switch Virtual Interfaces (SVI) is another method used for Inter VLAN Routing, commonly implemented on multilayer switches. Unlike Router-on-a-Stick topology, SVI does not require subinterfaces or a physical router connection for each VLAN. Instead, it uses virtual interfaces created directly on the switch to enable communication between VLANs. So, Switch Virtual Interfaces (SVI) is more efficient and scalable than Router-on-Stick topology.
If you want to learn How SVI Works and How to Configure SVI, you can check the below lessons:
What is SVI (Switch Virtual Interfaces)? | SVI Configuration with Cisco Packet Tracer
There are some differences between Router-on-Stick and SVIs. Below, you can find a comparison table as for Router-on-Stick and SVI .
| Feature | SVI (Switch Virtual Interface) | Router-on-a-Stick |
|---|---|---|
| Where routing happens | Inside Layer 3 switch | On router |
| Performance | High (hardware-based) | Lower (CPU-based) |
| Scalability | High | Limited |
| Configuration | Simple | More complex (subinterfaces) |
| Bottleneck | Very low | High (single trunk link) |
| Cost | Higher | Lower |
Inter VLAN Routing is the process of enabling communication between different VLANs using a Layer 3 device such as a router or a multilayer switch.
Because VLANs are separate broadcast domains. Devices in different VLANs cannot communicate without routing between them. Inter VLAN Routing provides this routing.
There are two main methods for Inter-VLAN Routing: Router-on-a-Stick topology and Switch Virtual Interfaces (SVI).
Router-on-a-Stick is a method where a single router interface is dividedinto multiple subinterfaces to route traffic between VLANs.
Router-on-a-Stick uses a router with subinterfaces, while SVI uses virtual interfaces directly on a multilayer switch for routing between VLANs.
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