How To Troubleshoot Router-on-a-Stick Inter-VLAN Routing Configuration.
In the router-on-a-stick routing topology below, the F0/3 interface on switch SW1 is not configured as a trunk and means the port is left in the default VLAN mode.
As the case may be, the router is not able to perform its routing function correctly because, each of its configured subinterfaces is unable to send or receive VLAN tagged traffic. This prevents all configured VLANs from routing through router R1 to reach the other VLANs.
To solve this problem, the switchport fa0/3 is to be configured as a trunk link. Use the switchport mode trunk interface configuration command on switch port F0/3 on switch SW1, doing this will enable connectivity between the switch SW1 and router R1. When the trunk is successfully established, devices connected to each of the VLANs are able to communicate with the subinterface assigned to their VLAN, allowing inter-VLAN routing to occur.