A company is expanding to another floor in the same building. The network engineer configures a new switch with the same VLANs as the existing stack. When the network engineer connects the new switch to the existing stack, all users lose connectivity. Which of the following is the MOST likely reason?

The symptoms described—a complete and immediate loss of connectivity for all users after adding a new switch—are characteristic of a Layer 2 switching loop. When a new switch is connected to an existing network, it can create a redundant physical path. If the Spanning Tree Protocol (STP) is misconfigured, disabled, or fails to converge properly, it will not block the redundant path. This creates a loop where broadcast and multicast frames are forwarded endlessly between switches. This phenomenon, known as a broadcast storm, rapidly consumes all available bandwidth and overwhelms the CPUs of the network devices, effectively bringing the entire network segment to a halt.

A. The new switch has unused ports disabled This is a security best practice that would only prevent new devices from connecting to those specific ports; it would not cause a network-wide outage for existing, connected users.
B. The new switch does not have a default gateway A default gateway is a Layer 3 setting used for remote management of the switch itself. Its absence does not affect the switch's primary Layer 2 function of forwarding frames for users.
C. The new switch is connected to an access port While connecting a switch to an access port is a misconfiguration (it should be a trunk port to carry multiple VLANs), this would typically isolate VLANs rather than cause a total network failure. It could, however, lead to an STP failure, making the resulting loop (Option D) the more direct cause of the outage.

1. Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. In Chapter 6, Section 6.4.2 "Link-Layer Switches," the text explains: "a topology with a cycle can create a situation in which frames loop forever... A packet can be forwarded in a cycle, with a new copy of the packet being created at each hop in the cycle. This proliferation of packets can bring a network to its knees." The solution described is the Spanning Tree Protocol.

2. Stanford University. (2019). CS144: Introduction to Computer Networking, Lecture 6: Switching. Slide 20, titled "What happens with cycles?", illustrates how broadcast frames are endlessly duplicated and forwarded in a looped topology, leading to a "broadcast storm" that cripples the network.

3. 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 foundational paper describes the problem of loops in bridged networks (p. 44, Section 1) and proposes the Spanning Tree Algorithm as the solution to prevent the resulting broadcast storms.

4. Cisco. (2023, October 26). Spanning Tree Protocol Problems and Related Design Considerations. In the section "Broadcast Storms," the document states: "A broadcast storm occurs when broadcast or multicast packets are flooded through a Layer 2 network... This can cause the network to slow down or become unusable." This official documentation explains the protocol's behavior in preventing such failures.