CCNP Infrastructure Practice Question
A company has a campus network with two distribution switches (DSW1 and DSW2) connected via a Layer 2 trunk. Each distribution switch connects to two access switches. Spanning Tree Protocol (STP) is running with default settings. Recently, a network administrator added a new access switch (ASW3) and connected it to both distribution switches. After the connection, network performance degraded significantly, and users in VLAN 10 reported intermittent connectivity. The administrator checked the logs and saw multiple TCN notifications. What is the most likely cause of the issue?
⚠ Common exam trap
Cisco often tests the distinction between a Layer 2 loop causing TCN flapping and a root bridge election, where candidates mistakenly think a new root bridge is the primary problem rather than the redundant physical loop itself.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The new switch is causing a Layer 2 loop due to redundant links without proper STP configuration.
When ASW3 is connected to both DSW1 and DSW2 via Layer 2 trunk links, it creates a physical loop in the network. With default STP settings, the new switch will participate in the spanning tree algorithm, but the sudden addition of redundant links can cause a temporary loop or instability until STP converges. The multiple TCN (Topology Change Notification) messages indicate that the spanning tree topology is flapping, leading to MAC address table flushes and intermittent connectivity for VLAN 10 users. This is the classic symptom of a Layer 2 loop caused by redundant links without proper STP configuration or before convergence completes.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The new switch is causing a Layer 2 loop due to redundant links without proper STP configuration.
Why this is correct
A Layer 2 loop occurs when redundant paths exist without Spanning Tree Protocol actively blocking one of them. The new switch, if connected with multiple links to the distribution switches and STP disabled or misconfigured, will forward broadcast frames out all ports, causing a broadcast storm. This leads to severe symptoms such as high CPU utilization on all switches, MAC address table flapping, and complete network unavailability.
- ✗
The new switch is not configured with the same VLANs as the distribution switches.
Why it's wrong here
A lack of identical VLAN configurations does not create loops; instead, it causes per-VLAN connectivity failures because traffic for missing VLANs is simply not forwarded. The switch can still run STP and maintain a loop-free topology for the VLANs it does carry. The described symptoms of broadcast storms and severe degradation are not consistent with a simple VLAN mismatch, which would affect only specific VLAN traffic.
- ✗
The new switch has a lower bridge priority than the current root bridge.
Why it's wrong here
Setting a lower bridge priority makes the new switch more eligible to become the root bridge, but STP still ensures a loop-free environment regardless of which switch is root. The root bridge is a legitimate role, and changing it merely alters the topology paths, causing some traffic to take different routes. It would not by itself generate the broadcast storms or MAC flapping indicative of a Layer 2 loop, so this is not the cause.
- ✗
The new switch has become the root bridge and is sending inferior BPDUs.
Why it's wrong here
The root bridge does not send inferior BPDUs; it sends superior BPDUs with a higher priority that other switches accept as the best. Becoming the root bridge is a normal condition and does not inherently create loops. This option is incorrect because it both mischaracterizes BPDU behavior and confuses a topology change with a loop, which is the actual cause.
Visual reference
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.