The answer is BPDU guard, the security feature that would have prevented the rogue switch from causing a network loop. BPDU guard works by monitoring access ports for any incoming Bridge Protocol Data Units (BPDUs), which are the frames switches use to negotiate the Spanning Tree Protocol (STP). If a BPDU is received on a port configured with BPDU guard, the switch immediately places that port into an errdisable state, effectively shutting it down and blocking the unauthorized device from participating in the network. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of STP security mechanisms, often appearing alongside PortFast and root guard as a common trap—students frequently confuse BPDU guard with root guard, but remember that BPDU guard reacts to any BPDU on an access port, while root guard protects the root bridge election. A solid memory tip: think of BPDU guard as a “bouncer” that kicks out any switch trying to sneak onto an access port, instantly stopping loops before they start.
N10-009 Network Security Practice Question
This N10-009 practice question tests your understanding of network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 security auditor discovers that an unauthorized switch has been connected to an access port in the wiring closet. The rogue switch caused a network loop and disrupted connectivity. Which security feature, if enabled on the access port, would have prevented this by disabling the port when a BPDU is received?
Correct. BPDU guard disables a port that receives a BPDU, effectively blocking unauthorized switches.
B
Root guard
Why wrong: Incorrect. Root guard prevents a port from becoming a root bridge but does not block BPDU reception entirely.
C
Loop guard
Why wrong: Incorrect. Loop guard prevents alternate ports from forwarding if BPDUs stop being received, but it does not disable the port upon receiving BPDUs.
D
UDLD
Why wrong: Incorrect. UDLD detects unidirectional links but does not prevent BPDU-related loops from rogue switches.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
BPDU guard
BPDU guard is the correct answer because it is specifically designed to protect against rogue switch connections on access ports. When enabled, if a port receives any Bridge Protocol Data Unit (BPDU), it immediately places the port into an errdisable state, effectively disabling it and preventing a potential network loop. This directly addresses the scenario where an unauthorized switch connected to an access port caused a loop.
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.
✓
BPDU guard
Why this is correct
Correct. BPDU guard disables a port that receives a BPDU, effectively blocking unauthorized switches.
Related concept
Read the scenario before looking for a memorised answer.
✗
Root guard
Why it's wrong here
Incorrect. Root guard prevents a port from becoming a root bridge but does not block BPDU reception entirely.
✗
Loop guard
Why it's wrong here
Incorrect. Loop guard prevents alternate ports from forwarding if BPDUs stop being received, but it does not disable the port upon receiving BPDUs.
✗
UDLD
Why it's wrong here
Incorrect. UDLD detects unidirectional links but does not prevent BPDU-related loops from rogue switches.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between BPDU guard and Root guard, where candidates mistakenly choose Root guard thinking it prevents loops, but Root guard only protects the root bridge election and does not disable a port upon BPDU reception.
Detailed technical explanation
How to think about this question
BPDU guard is typically configured on PortFast-enabled access ports, where PortFast bypasses the normal listening and learning states to bring a port up quickly. The guard works by monitoring for any BPDU arrival; if one is received, it assumes an unauthorized switch is connected and errdisables the port. This behavior is defined in IEEE 802.1D and Cisco's implementation, and the errdisable state can be manually recovered or automatically recovered via errdisable recovery mechanisms. In real-world scenarios, BPDU guard is a critical first line of defense against accidental or malicious STP manipulation in the access layer.
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 N10-009 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 N10-009 question in full detail.
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: BPDU guard — BPDU guard is the correct answer because it is specifically designed to protect against rogue switch connections on access ports. When enabled, if a port receives any Bridge Protocol Data Unit (BPDU), it immediately places the port into an errdisable state, effectively disabling it and preventing a potential network loop. This directly addresses the scenario where an unauthorized switch connected to an access port caused a loop.
What should I do if I get this N10-009 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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