When enabling inter-VLAN routing on a router, one of the most common configuration errors is to connect the physical router interface to the wrong switch port. This places the router interface in the incorrect VLAN and prevents it from reaching the other devices within the same subnet.

As shown in the figure, router R1 interface G0/0 is connected to switch S1 port F0/9. Switch port F0/9 is configured for the default VLAN, not VLAN 10. This prevents PC1 from being able to communicate with the router interface. Therefore, it is unable to route to VLAN 30.

To correct this problem, physically connect the router R1 interface G0/0 to switch S1 port F0/4. This puts the router interface in the correct VLAN and allows inter-VLAN routing. Alternately, change the VLAN assignment of switch port F0/9 to VLAN 10. This also allows PC1 to communicate with router R1 interface G0/0.