The described symptoms—network loops and unacceptable recovery time after switch failures—point directly to a misconfigured or absent loop-prevention mechanism in a network with redundant physical paths. The Spanning Tree Protocol (STP) is a Layer 2 protocol specifically designed to prevent these issues. It creates a loop-free logical topology by blocking redundant links and automatically unblocks them if a primary link fails, thus providing path redundancy and improving recovery time. Since STP is a standard feature on managed switches, implementing it is a configuration change, making it the most cost-effective solution that directly addresses the root cause.

A. Add a new Layer 3 switch.

A Layer 3 switch provides routing, which operates at a different network layer and does not resolve Layer 2 loops within a broadcast domain. This is also a costly hardware addition.

B. Add multiple VLANs.

VLANs segment a network into separate broadcast domains, which can contain a loop's impact to a single VLAN but does not prevent the loop from forming within that VLAN.

D. Implement tagging.

Tagging (e.g., IEEE 802.1Q) is a mechanism for identifying VLAN traffic on trunk links between switches. It is not a loop-prevention protocol.

1. IEEE Std 802.1D-2004

IEEE Standard for Local and metropolitan area networks—Media Access Control (MAC) Bridges. Section 17

"Spanning Tree Protocol

" details the protocol's operation for creating a single

active

loop-free path in a bridged network. The introduction (Section 1) states the standard's purpose is to ensure interoperability for "the provision of loop-free paths."

2. Kurose

J. F.

& Ross

K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In Section 6.4.3

"Link-Layer Switches

" the text explains: "To deal with the problem of loops in redundant topologies

switches use a spanning tree protocol... This protocol allows switches to create a spanning tree from the redundant topology

by disabling a subset of the links."

3. McKeown

N. (2013). CS144: Introduction to Computer Networking

Lecture 10: Switching. Stanford University. Slides 22-30 describe the Spanning Tree Algorithm as the solution to "prevent loops" in switched Ethernet networks by having switches collectively decide which links to use and which to turn off to form a loop-free spanning tree.

4. Cisco Systems

Inc. (2022). Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches. Document ID: 10556. The introduction states

"The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network... STP operates on all switches and creates a loop-free topology."