Switching and loop prevention (Layer 2): Which of the following is a Layer 2 control protocol specifically designed to maintain a loop-free topology in an Ethernet switched network?

Introduction / Context:
The classic problem in switched Ethernet LANs is that redundant links, while good for availability, create Layer 2 loops. These loops can cause broadcast storms and MAC address table instability. The industry solution is the Spanning Tree Protocol (STP), a Layer 2 control protocol that logically blocks some paths to guarantee a single active tree.

Given Data / Assumptions:

• We are discussing Layer 2 (Data Link) control behavior, not Layer 3 routing.
• The environment is an Ethernet switched LAN with potential redundant links.
• We want the protocol whose purpose is loop prevention at Layer 2.

Concept / Approach:
STP elects a root bridge, calculates the shortest path tree using path cost, and places nonessential ports into a blocking (or alternate) state. Variants include 802.1D (original), 802.1w (Rapid STP), and 802.1s (Multiple STP). Other protocols listed serve different roles (discovery, VLAN management, routing).

Step-by-Step Solution:

Verification / Alternative check:
Vendor docs and IEEE 802.1D/W/S define STP/RSTP/MSTP as the loop-avoidance mechanisms. No other listed protocol performs this function at Layer 2.

Why Other Options Are Wrong:

• VTP: Manages VLANs across Cisco switches; does not block loops.
• RIP: Layer 3 distance-vector routing; not a Layer 2 protocol.
• CDP/LLDP: Neighbor discovery; informational only.

Common Pitfalls:
Confusing VTP (VLAN sharing) with STP; assuming link aggregation replaces STP (bundles still need loop control across topologies).

Final Answer:
STP