Definition - What does One-Armed Router mean?
A one-armed router takes care of the data traffic distribution between virtual local area networks (VLANs). Like a normal router, it transports and distributes data to their proper destination. However, the one-armed router only moves traffic within the same physical network since VLANs and the routers within them are primarily used for security purposes.
Techopedia explains One-Armed Router
One-armed routers connect virtual systems that are intended to encourage or discourage the connectivity between a standard LAN and multiple devices.
Even though the usual VLAN consists of multiple networks that perform in the same physical space, they are still considered as separate networks, which means they need a way to communicate with each other. One-armed routers have a single network interface controller which is recognized by every virtual network that connects to it, so the information from the different VLANs can get to its destination from any connected network, VLAN or node.
They operate using the 80/20 rule which means that 80% of the network traffic stays in the VLAN and does not need further assistance from the one-armed router. The other 20% of the network traffic consists of communication between multiple VLANs which goes through the one-armed router.
One-armed routers take care of the intensive traffic between VLANs, so they must free up the primary data path in the network to facilitate inter-VLAN traffic. A major disadvantage of the one-armed router structure is that it can be the cause of bottlenecks if there is too much traffic between VLANs, which also can develop into a single point of network failure.