The answer is to enable BPDU Guard on all access ports. This feature immediately error-disables any port that receives a BPDU, which should never occur on a properly configured access port, thereby preventing loops caused by an unauthorized switch connecting and injecting BPDUs into the Rapid PVST+ topology. On the ENCOR 350-401 exam, this scenario tests your understanding of STP security mechanisms, often contrasting BPDU Guard with Root Guard or Loop Guard; a common trap is confusing BPDU Guard (which reacts to any BPDU) with Root Guard (which only reacts to superior BPDUs). Remember the memory tip: "Guard the edge—disable on a BPDU pledge," meaning BPDU Guard protects the network edge by shutting down a port the moment it hears a BPDU.
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 that occurred in a network using Rapid PVST+. The network has three switches: SW1 (root), SW2, and SW3. The engineer examines the topology and finds that SW2 and SW3 are connected via a link that is not supposed to be there. The engineer suspects that an unauthorized switch was connected to the network, causing the loop. The engineer wants to prevent such loops in the future by configuring a feature that will disable any port that receives a BPDU from an unauthorized switch. Which feature should the engineer configure on the access ports?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable BPDU Guard on all access ports.
BPDU Guard is the correct feature because it immediately error-disables a port when a BPDU is received, preventing loops from unauthorized switches. Since the engineer wants to protect access ports from receiving BPDUs (which should never occur on a properly configured access port), BPDU Guard directly addresses the scenario of an unauthorized switch being connected and sending BPDUs.
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 BPDU Guard on all access ports.
Why this is correct
Correct because BPDU Guard disables a port if a BPDU is received, preventing unauthorized switches from causing loops.
Related concept
Read the scenario before looking for a memorised answer.
✗
Enable Loop Guard on all access ports.
Why it's wrong here
Incorrect because Loop Guard prevents loops when BPDUs are not received, but it does not disable the port upon receiving BPDUs.
✗
Enable Root Guard on all access ports.
Why it's wrong here
Incorrect because Root Guard prevents a port from becoming a root port, but it does not disable the port upon receiving BPDUs.
✗
Enable UDLD on all access ports.
Why it's wrong here
Incorrect because UDLD detects unidirectional links, but it does not prevent loops caused by unauthorized switches.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between BPDU Guard and Root Guard, where candidates mistakenly choose Root Guard because they think it protects against unauthorized switches, but Root Guard only prevents a port from becoming root, not from receiving BPDUs and causing loops.
Detailed technical explanation
How to think about this question
BPDU Guard works by monitoring ports configured with PortFast; when a BPDU is received, the port is immediately placed into an errdisable state, effectively blocking all traffic. This feature is typically configured globally with 'spanning-tree portfast bpduguard default' or per interface with 'spanning-tree bpduguard enable', and it is essential for access ports where no BPDUs should ever appear. In a real-world scenario, if an attacker connects a rogue switch to an access port, BPDU Guard will disable that port within milliseconds, preventing the STP topology from being altered and avoiding a Layer 2 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 practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
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 BPDU Guard on all access ports. — BPDU Guard is the correct feature because it immediately error-disables a port when a BPDU is received, preventing loops from unauthorized switches. Since the engineer wants to protect access ports from receiving BPDUs (which should never occur on a properly configured access port), BPDU Guard directly addresses the scenario of an unauthorized switch being connected and sending BPDUs.
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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Question Discussion
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