CCNA Switching and Network Access Practice Question
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?
⚠ Common exam trap
Cisco often tests the distinction between Loop Guard (reacts to BPDU loss) and UDLD (detects unidirectional links via proprietary keepalives), where candidates mistakenly choose UDLD because the question mentions a unidirectional fiber cut, but the key clue is the specific 'loop-inconsistent' state, which is unique to Loop Guard.
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 a Spanning Tree Protocol (STP) enhancement that prevents alternate or root ports from becoming designated forwarding ports when BPDUs are no longer received. When a unidirectional fiber cut causes BPDU loss on a root port, Loop Guard transitions the port to the loop-inconsistent state, blocking traffic until BPDUs are received again. This matches the scenario exactly, making option B correct.
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 is an ingress protection feature used on PortFast-enabled access ports; it immediately error-disables a port upon receiving any BPDU, regardless of whether that BPDU is superior, inferior, or expected. It does not monitor for the absence or loss of BPDUs, so a unidirectional link that stops sending BPDUs would not trigger BPDU Guard. Consequently, BPDU Guard cannot produce the loop-inconsistent STP state described in the scenario.
- ✓
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.
- ✗
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?”
Visual reference
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Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
Rapid Spanning Tree Protocol
Rapid Spanning Tree Protocol (RSTP) is a network protocol that prevents loops in Ethernet networks by quickly recalculating the best path when a link fails.
Key term
cut
In scripting and automation, cut is a command-line utility used to extract specific fields or columns from each line of a file or text stream based on a delimiter or character position.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.