A one-armed router is a router that routes traffic between virtual local area networks (VLANs). A one-armed router operates on the 80/20 rule, which states that 80% of traffic in a network remains within a virtual local area network and doesn't need routing service. The other 20% of network traffic is between VLANs and goes through the one-armed router. Because the one-armed router takes care of the more intensive traffic between VLANs, it frees the primary data path in a network for inter-VLAN traffic.
In order for a one-armed router to be beneficial, the VLAN must be configured to the 80/20 rule. One disadvantage of using the one-armed router structure is that it represents a single point of failure in a network. Another disadvantage is it can develop into a bottleneck if there are large amounts of traffic between VLANs.