Unlike legacy interVLAN routing, the router-on-a-stick scenario (see Figure 6.8) of the interVLAN routing concept uses a different approach to enable communication among VLANs. The router establishes a trunk link with the switch where the VLANs are configured. Then, on a router side, the subinterfaces are configured with an appropriate IP address in a way that each subinterface represents a separate subnet, maps a specific VLAN, and tags a frame for the VLANs. In addition, it requires assigning the IP addresses in PCs' interfaces according to the corresponding subnets of the subinterfaces where they do belong.
After all that configuration, the router can then perform communication among VLANs by traversing the trunk link. In this section, router-on-a-stick interVLAN routing on a Cisco router is explained:
