What is a primary characteristic of the Spanning-Tree Protocol?

The primary characteristic of the Spanning-Tree Protocol (STP) is its ability to prevent network loops. STP is designed to maintain a loop-free topology in Ethernet networks, which is crucial because network loops can lead to broadcast storms, multiple frame copies, and ultimately network congestion or failure. By intelligently managing the active paths in a Layer 2 network, STP determines which ports on a switch should be in forwarding or blocking states, effectively creating a single active path between devices.

STP operates at Layer 2 of the OSI model, not Layer 3, thus ruling out the first option. While STP primarily runs on switches to manage Ethernet connections rather than routing traffic, it does not run on routers in the same context as it does on switches. Additionally, STP is not disabled by default on switches; it is enabled to ensure loop prevention right from the start. This understanding reflects the critical role that STP plays in maintaining stability and reliability in network topologies.