- A
Enable Loop Guard on all switch ports.
Correct because Loop Guard prevents loops by keeping a port in blocking state if BPDUs are not received, ensuring that a port does not transition to forwarding incorrectly.
- B
Enable BPDU Guard on all switch ports.
Why wrong: Incorrect because BPDU Guard is used to protect access ports from receiving BPDUs, not to prevent loops during topology changes.
- C
Enable Root Guard on all switch ports.
Why wrong: Incorrect because Root Guard prevents a port from becoming a root port, but it does not prevent loops during topology changes.
- D
Enable UDLD on all fiber links.
Why wrong: Incorrect because UDLD detects unidirectional links, but it does not prevent loops that occur due to topology changes in a properly functioning network.
Quick Answer
The answer is to enable Loop Guard on all switch ports. This feature prevents temporary loops during Rapid PVST+ convergence by keeping a port in a blocking state if BPDUs stop arriving, which is exactly what happens after a link failure in a triangle topology like the one described. When the root port fails, the switch may prematurely transition an alternate port to forwarding before the new root port synchronizes, creating a loop; Loop Guard stops this by requiring continuous BPDU reception before allowing a transition. On the ENCOR 350-401 exam, this scenario tests your understanding of how Rapid PVST+ handles topology changes differently from classic STP, and a common trap is confusing Loop Guard with UDLD or Root Guard—remember that Loop Guard specifically addresses the “BPDU starvation” condition that causes temporary loops. A helpful memory tip: “No BPDU? No forward—Loop Guard holds the port.”
350-401 Spanning Tree Protocol Practice Question
This 350-401 practice question tests your understanding of spanning tree protocol. 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 network engineer is troubleshooting a Layer 2 loop issue. The network consists of three switches: SW1, SW2, and SW3, all connected in a triangle. The engineer notices that SW1 is the root bridge. After a link failure between SW1 and SW2, the network experiences a temporary loop. The engineer wants to prevent such loops in the future by enabling a feature that provides faster convergence and prevents temporary loops during topology changes. The engineer is using Rapid PVST+. Which feature should the engineer enable?
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
Enable Loop Guard on all switch ports.
When a link fails in a triangle topology with Rapid PVST+, the switch that lost its root port may temporarily transition a blocked alternate port to forwarding before the new root port is fully synchronized, causing a loop. Enabling Loop Guard on all switch ports prevents this by keeping a port in a blocking state if BPDUs are not received, ensuring that a port does not erroneously transition to forwarding during a topology change. This provides faster convergence without temporary loops by enforcing BPDU-based loop prevention.
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.
- ✓
Enable Loop Guard on all switch ports.
Why this is correct
Correct because Loop Guard prevents loops by keeping a port in blocking state if BPDUs are not received, ensuring that a port does not transition to forwarding incorrectly.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Enable BPDU Guard on all switch ports.
Why it's wrong here
Incorrect because BPDU Guard is used to protect access ports from receiving BPDUs, not to prevent loops during topology changes.
- ✗
Enable Root Guard on all switch ports.
Why it's wrong here
Incorrect because Root Guard prevents a port from becoming a root port, but it does not prevent loops during topology changes.
- ✗
Enable UDLD on all fiber links.
Why it's wrong here
Incorrect because UDLD detects unidirectional links, but it does not prevent loops that occur due to topology changes in a properly functioning network.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse Loop Guard with UDLD or BPDU Guard, thinking that any loop-prevention feature will solve temporary loops, but only Loop Guard directly addresses the scenario where a port transitions to forwarding due to loss of BPDUs during a topology change.
Detailed technical explanation
How to think about this question
Loop Guard works by monitoring the receipt of BPDUs on non-designated ports; if BPDUs stop arriving (e.g., due to a link failure or unidirectional link), the port is moved into a loop-inconsistent state (blocking) rather than transitioning to forwarding. In Rapid PVST+, the convergence relies on proposal/agreement handshakes, and Loop Guard ensures that a port does not become designated prematurely when BPDUs are absent, which is critical in triangle topologies where alternate ports exist. A real-world scenario is when a fiber link fails but the switch still receives traffic due to a hardware fault, Loop Guard prevents the alternate port from forwarding and causing a loop.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 350-401 question test?
Spanning Tree Protocol — This question tests Spanning Tree Protocol — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable Loop Guard on all switch ports. — When a link fails in a triangle topology with Rapid PVST+, the switch that lost its root port may temporarily transition a blocked alternate port to forwarding before the new root port is fully synchronized, causing a loop. Enabling Loop Guard on all switch ports prevents this by keeping a port in a blocking state if BPDUs are not received, ensuring that a port does not erroneously transition to forwarding during a topology change. This provides faster convergence without temporary loops by enforcing BPDU-based loop prevention.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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