Which protocol runs on layer 2 devices such as switches and bridges to ensure loop-free communications?

• A. Spanning Tree Protocol (STP)
• B. Spam
• C. Spear-Phishing
• D. Spyware

Answer: (A)

The concept being tested is how networks prevent loops when there are redundant paths at Layer 2. Spanning Tree Protocol runs on switches and bridges to create a loop-free forwarding topology. It does this by electing a root bridge and designating a single active path between devices, while other parallel paths are put into a blocking state so they don’t forward frames. Bridges exchange BPDU messages to compare path costs and determine which ports should forward and which should block. If a link fails, STP can reconverge and activate a previously blocked path to maintain connectivity, preserving redundancy without loops. Variants like RSTP and MSTP improve convergence but keep the core idea: build a spanning tree that ensures only one active path between any two devices. The other choices aren’t about managing Layer 2 topology; they refer to spam, spear-phishing, or spyware, which are unrelated to loop prevention.