Inter VLAN routing overview

When a Layer 2 switch divides a network by VLANs, communication between different VLANs becomes impossible. You can freely decide how you want to divide the network. As a general example, in a company’s internal network, the network is often divided into departments, and if the network is divided into departments using VLANs and communication between the departments becomes impossible, it will naturally interfere with business operations. Since we are in the same company, we also need to communicate between departments, i.e., between VLANs.

If using a router, then router is dividing the network and interconnecting at the same time. Therefore, communication between different networks is possible. To enable communication between VLANs, VLANs must be interconnected, which is called “inter-VLAN routing”.

Layer 3 network devices such as routers and layer 3 switches are required to implement inter-VLAN routing.

A simple example of inter-VLAN routing

Let us consider a simple example of inter-VLAN routing. The following figure shows an example where an internal network is divided into three networks for each department by creating three VLANs with Layer 2 switch. The networks divided by the VLANs of the Layer 2 switch are not interconnected. Therefore, communication between VLANs, or departments, is not possible. Therefore, VLANs are interconnected by routers or Layer 3 switches. Then, communication between VLANs can be done through a router or a layer 3 switch.

Figure Inter VLAN routing overview
Figure Inter VLAN routing overview
In order to communicate between VLANs, it is assumed that the TCP/IP configuration of the hosts as well as the routing configuration in the routers and layer 3 switches that connect the VLANs to each other are correctly configured.