Legacy inter-VLAN routing needed the physical interface for each VLAN, and the router has a limited number of physical interfaces. So, the use of Legacy Inter-VLAN routing is very limited. As the number of VLAN increased on a network, more physical interfaces needed. So, this configuration is not possible to use in the large network. So, the next solution for up to 50 VLAN is Router-on-a-Stick configuration. The router on Stick configuration uses VLAN trunking and sub-interfaces. As we learned in the previous article that VLAN trunking allows a single physical router interface to route traffic for many VLANs. This technique overcomes the hardware limitations based on physical router interfaces. The figure below illustrates the Router-on-Stick configuration.
When configuring inter-VLAN routing using the router-on-a-stick model, the connected switch port must be configured as a trunk. On the router, subinterfaces for each unique VLAN on the network needed and must be assigned an IP address specific to its subnet/VLAN and is also configured to tag frames for that VLAN. So we are going to configure Router-on-Stick inter-VLAN routing.
Configure Router-on-a-Stick – Switch
The Router-on-Stick configuration needed Trunk link connected to the physical interface of the Router. The Figure above illustrates, that the Switch port G0/1 connected to the physical interface of the router. So to enable inter-VLAN routing using router-on-a-stick, configure the trunk. Following is the switch configuration for this model of inter-VLAN routing.
Configure Router-on-a-Stick – Router
We just configure the switch for a router on stick model. So, let us configure the router sub-interfaces for this model. We have two VLANs on the switch, So, we need two sub-interfaces. The figure below illustrates the configuration of the router for the router-on-stick model.
We have configured two sub-interfaces according to the VLAN structure. Now both VLAN can communicate with each other. You can see the video for configuration.