Different VLANs in layer 2 switches cannot access each other, so it is necessary to add an extra router for one-arm routing to realize communication between different VLANs on the same switch. For example, vlan 1 accesses vlan2 through routing to realize communication between different vlans, which is helpful to understand and learn vlan principle and subinterface concept. The disadvantage is that it is easy to become a single point of network failure, the configuration is slightly complicated, and the practical significance is not great.

Router-on-a-stick refers to the way of configuring subinterfaces (or "logical interfaces" without real physical interfaces) on one interface of the router to realize the interconnection between different VLAN (virtual local area networks) that were originally isolated from each other.