The answer is a Layer 2 loop caused by redundant links without proper STP convergence. When the new access switch ASW3 connects to both distribution switches via trunk links, it creates a physical loop in the topology. With default STP settings, the sudden addition of these redundant paths triggers repeated Topology Change Notification (TCN) flapping as the spanning tree algorithm struggles to stabilize. Each TCN forces switches to flush their MAC address tables, which explains the intermittent connectivity for VLAN 10 users—the classic symptom of a loop. On the ENCOR 350-401 exam, this scenario tests your understanding of STP convergence behavior and the impact of TCNs on network stability. A common trap is assuming the issue is a misconfigured portfast or a VLAN mismatch, but the core problem is the loop itself. Memory tip: TCN flapping equals MAC table flushing equals intermittent connectivity—think “TCN = Table Churn Now.”
CCNP Infrastructure Practice Question
This 350-401 practice question tests your understanding of infrastructure. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
Redundant links without proper STP can cause loops.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The new switch is not configured with the same VLANs as the distribution switches.
The new switch has a lower bridge priority than the current root bridge.
Why it's wrong here
This would cause a root bridge change, but not necessarily the severe symptoms described.
✗
The new switch has become the root bridge and is sending inferior BPDUs.
Why it's wrong here
Becoming root bridge is not inherently a problem.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
STP uses BPDUs to elect a root bridge and determine which ports should be in forwarding or blocking state. When a new switch is added with redundant links, the network experiences a topology change, triggering TCNs that cause all switches to flush their MAC address tables temporarily. This flooding of unknown unicast frames can overwhelm the network, especially if the loop is not immediately blocked due to STP convergence delays (forward delay timers of 15 seconds each for listening and learning). In a real-world scenario, if PortFast is not enabled on access ports, the addition of a new switch can cause extended disruption as STP recalculates.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Infrastructure — This question tests Infrastructure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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