- A
BPDU Guard is enabled on the port, causing it to be placed in error-disabled state.
Why wrong: BPDU Guard would immediately error-disable a port if any BPDU is received, typically used on PortFast-enabled access ports. It does not react to a loss of BPDUs and would not produce a loop-inconsistent state.
- B
Loop Guard is active on the root port and transitioned it to loop-inconsistent state upon BPDU loss.
Loop Guard is precisely designed to monitor BPDU reception on blocked or alternate ports. When a unidirectional link failure occurs and BPDUs are no longer received, Loop Guard places the port into the loop-inconsistent state, blocking all traffic to prevent a potential loop. The 'loop-inconsistent' state is a clear indicator of this feature.
- C
UDLD has detected a unidirectional link and has shut down the port.
Why wrong: UDLD (UniDirectional Link Detection) can detect one-way link failures on fiber connections and will place the port in an err-disabled state or shut it down, not in a loop-inconsistent state. Loop-inconsistent is specific to STP Loop Guard.
- D
Root Guard is preventing the port from transitioning to designated forwarding after losing BPDUs.
Why wrong: Root Guard is configured on designated ports to enforce the root bridge placement by blocking a port that receives superior BPDUs. It does not react to a loss of BPDUs on a root port, nor does it produce a loop-inconsistent state.
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. 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 network engineer notices that a root port on a switch has transitioned to a loop-inconsistent state. The port was previously receiving BPDUs normally, but after a suspected unidirectional fiber cut, it no longer receives BPDUs. What is the most likely cause?
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 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
Loop Guard is active on the root port and transitioned it to loop-inconsistent state upon BPDU loss.
Loop Guard is an STP enhancement that monitors the reception of BPDUs on a blocked port. When BPDUs stop arriving (due to a unidirectional link failure), Loop Guard moves the port to loop-inconsistent state, preventing it from transitioning to the forwarding state and thus avoiding a switching loop.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 is enabled on the port, causing it to be placed in error-disabled state.
Why it's wrong here
BPDU Guard would immediately error-disable a port if any BPDU is received, typically used on PortFast-enabled access ports. It does not react to a loss of BPDUs and would not produce a loop-inconsistent state.
- ✓
Loop Guard is active on the root port and transitioned it to loop-inconsistent state upon BPDU loss.
Why this is correct
Loop Guard is precisely designed to monitor BPDU reception on blocked or alternate ports. When a unidirectional link failure occurs and BPDUs are no longer received, Loop Guard places the port into the loop-inconsistent state, blocking all traffic to prevent a potential loop. The 'loop-inconsistent' state is a clear indicator of this feature.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
UDLD has detected a unidirectional link and has shut down the port.
Why it's wrong here
UDLD (UniDirectional Link Detection) can detect one-way link failures on fiber connections and will place the port in an err-disabled state or shut it down, not in a loop-inconsistent state. Loop-inconsistent is specific to STP Loop Guard.
- ✗
Root Guard is preventing the port from transitioning to designated forwarding after losing BPDUs.
Why it's wrong here
Root Guard is configured on designated ports to enforce the root bridge placement by blocking a port that receives superior BPDUs. It does not react to a loss of BPDUs on a root port, nor does it produce a loop-inconsistent state.
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Loop Guard is active on the root port and transitioned it to loop-inconsistent state upon BPDU loss.Correct answer▾
Why this is correct
Loop Guard is precisely designed to monitor BPDU reception on blocked or alternate ports. When a unidirectional link failure occurs and BPDUs are no longer received, Loop Guard places the port into the loop-inconsistent state, blocking all traffic to prevent a potential loop. The 'loop-inconsistent' state is a clear indicator of this feature.
✗BPDU Guard is enabled on the port, causing it to be placed in error-disabled state.Wrong answer — click to see why▾
Why this is wrong here
BPDU Guard is a protective feature that disables a port upon receiving a BPDU, not upon losing BPDUs. The symptom here is a loss of BPDUs, not a reception of unexpected BPDUs.
✗UDLD has detected a unidirectional link and has shut down the port.Wrong answer — click to see why▾
Why this is wrong here
UDLD acts by shutting down the port or putting it in errdisable state, while the scenario explicitly shows the port in a loop-inconsistent state, indicating an STP-based protection mechanism.
✗Root Guard is preventing the port from transitioning to designated forwarding after losing BPDUs.Wrong answer — click to see why▾
Why this is wrong here
Root Guard would block a port if it received a BPDU with better root information, not when BPDUs stop arriving. It also does not produce a loop-inconsistent state.
Analysis generated from the official 200-301blueprint 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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 200-301 NAT questions on configuration and troubleshooting.
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Switching and Network Access — study guide chapter
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FAQ
Questions learners often ask
What does this 200-301 question test?
Switching and Network Access — This question tests Switching and Network Access — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Loop Guard is active on the root port and transitioned it to loop-inconsistent state upon BPDU loss. — Loop Guard is an STP enhancement that monitors the reception of BPDUs on a blocked port. When BPDUs stop arriving (due to a unidirectional link failure), Loop Guard moves the port to loop-inconsistent state, preventing it from transitioning to the forwarding state and thus avoiding a switching loop.
What should I do if I get this 200-301 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 200-301 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 14, 2026
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.
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