Which protocol is used to detect loops in switching?

Spanning Tree Protocol (STP), defined by the IEEE 802.1D standard, is a Layer 2 network protocol designed to prevent logical loops in a redundant switched topology. When multiple paths exist between switches, STP creates a loop-free logical topology by selectively blocking redundant paths. This prevents broadcast storms and MAC address table instability that would otherwise occur in a looped Layer 2 network. The protocol achieves this by electing a root bridge and calculating the shortest path from all other switches to it, placing any redundant links into a blocking state.

B. RIP: Routing Information Protocol (RIP) is a Layer 3 distance-vector routing protocol used to exchange routing information; it does not operate at the switching layer to detect loops.

C. OSPF: Open Shortest Path First (OSPF) is a Layer 3 link-state routing protocol; its function is to find the best path for IP packets, not to prevent Layer 2 switching loops.

D. BGP: Border Gateway Protocol (BGP) is a Layer 3 path-vector protocol used for routing between autonomous systems; it is unrelated to Layer 2 loop detection within a LAN.

1. Official Vendor Documentation: Cisco Systems, "Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches," Document ID: 10556. This document states, "Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on bridges and switches. The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network."

2. Academic Publication: Perlman, R. (1985). "An Algorithm for Distributed Computation of a Spanning Tree in an Extended LAN." ACM SIGCOMM Computer Communication Review, 15(4), 44–53. https://doi.org/10.1145/318951.319004. This paper is the original work by the inventor of STP, detailing the algorithm's design for creating a loop-free spanning tree in a bridged/switched network.

3. Official Vendor Documentation: Cisco Press, "LAN Switching and Wireless, CCNA Exploration Companion Guide" (2008), Chapter 1, "LAN Design," Section: "Redundancy at OSI Layers 1 and 2," p. 14. The text explains, "Without a mechanism such as Spanning Tree Protocol (STP) to block redundant physical paths, Layer 2 loops can form. A Layer 2 loop can result in broadcast storms..."

4. Official Vendor Documentation: Cisco Systems, "IP Routing: RIP Configuration Guide, Cisco IOS Release 15M&T," Chapter: "Configuring a Basic RIP Network." This guide defines RIP as a routing protocol for IP networks, confirming its operation at Layer 3, distinct from Layer 2 switching.