Router on a Stick (ROAS)

The subinterface number does not have to match the VLAN number. However, it is highly recommended that they do match, to make it easier to understand.

ROAS is used to route between multiple VLANs using a single interface on the router and switch.

The switch interface is configured as a regular trunk.

The router interface is configured using subinterfaces. You configure the VLAN tag and IP address on each subinterface.

The router will behave as if frames arriving with a certain VLAN tag have arrived on the subinterface configured with that VLAN tag.

The router will tag frames sent out of each subinterface with the VLAN tag configured on the subinterface.