- A
BPDU guard
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.
Question 502 of 520
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?
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
✓
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.
When this WOULD be correct
Root guard would be correct in a scenario where an administrator wants to ensure that a specific switch remains the root bridge and prevent any other switch from becoming root by sending superior BPDUs. For example, on a port connected to a customer switch, root guard would disable the port if a superior BPDU is received.
- ✗
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.
When this WOULD be correct
A question describing a scenario where a unidirectional link failure causes a loop, and the solution must prevent a port from transitioning to forwarding state when BPDUs stop arriving (e.g., due to a faulty cable).
- ✗
UDLD
Why it's wrong here
Incorrect. UDLD detects unidirectional links but does not prevent BPDU-related loops from rogue switches.
When this WOULD be correct
UDLD would be correct in a scenario where a network administrator needs to detect and disable ports that are only transmitting in one direction, preventing issues like routing loops or STP failures due to unidirectional links.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓BPDU guardCorrect answer▾
Why this is correct
Correct. BPDU guard disables a port that receives a BPDU, effectively blocking unauthorized switches.
✗Root guardWrong answer — click to see why▾
Why this is wrong here
Root guard is used to enforce the root bridge position in a spanning-tree topology, not to disable ports upon receiving BPDUs. It does not prevent unauthorized switches from causing loops by disabling the port.
★ When this WOULD be the correct answer
Root guard would be correct in a scenario where an administrator wants to ensure that a specific switch remains the root bridge and prevent any other switch from becoming root by sending superior BPDUs. For example, on a port connected to a customer switch, root guard would disable the port if a superior BPDU is received.
Why candidates choose this
Candidates may confuse root guard with BPDU guard because both involve BPDUs and port disabling, but root guard focuses on maintaining root bridge status rather than preventing unauthorized switches.
✗Loop guardWrong answer — click to see why▾
Why this is wrong here
Loop guard is designed to prevent alternate or root ports from becoming designated ports in the absence of BPDUs, not to disable a port upon receiving a BPDU. It does not protect against rogue switches sending BPDUs.
★ When this WOULD be the correct answer
A question describing a scenario where a unidirectional link failure causes a loop, and the solution must prevent a port from transitioning to forwarding state when BPDUs stop arriving (e.g., due to a faulty cable).
Why candidates choose this
Candidates may confuse loop guard with BPDU guard because both deal with loops and BPDUs, but loop guard addresses missing BPDUs rather than unexpected BPDUs.
✗UDLDWrong answer — click to see why▾
Why this is wrong here
UDLD (Unidirectional Link Detection) detects unidirectional links but does not disable a port upon receiving BPDUs; it is not designed to prevent rogue switches or loops caused by BPDU reception.
★ When this WOULD be the correct answer
UDLD would be correct in a scenario where a network administrator needs to detect and disable ports that are only transmitting in one direction, preventing issues like routing loops or STP failures due to unidirectional links.
Why candidates choose this
Candidates may confuse UDLD with BPDU guard because both involve loop prevention, but UDLD focuses on unidirectional links rather than unauthorized BPDU reception.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this N10-009 question test?
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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Last reviewed: Jun 30, 2026
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