Quick Answer
The answer is a faulty SFP or optical cable. CRC errors on a Fibre Channel link indicate physical-layer corruption of frames, typically caused by faulty optics, dirty or damaged fiber cables, or marginal signal integrity; when excessive bit errors accumulate, the link loses synchronization and flaps as the port repeatedly resets or re-initializes. On the Cisco DCCOR and CCNP Data Center Core 350-601 exam, this scenario tests your ability to distinguish physical-layer issues from protocol or configuration problems—a common trap is chasing Fibre Channel zoning or buffer-to-buffer credit errors when the real culprit is a bad transceiver. Remember the memory tip: CRC = Cable or Receiver Corruption.
This 350-601 practice question tests your understanding of storage network. 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.
An engineer notices that a Fibre Channel link between two Cisco MDS 9000 series switches is flapping every few minutes. The interface counters show a high number of CRC errors. What is the most likely cause of this issue?
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.
NPV mode enabled on the switch
Why wrong: NPV mode does not cause CRC errors.
Incorrect zoning configuration
Why wrong: Zoning affects access control, not physical errors.
Faulty SFP or optical cable
CRC errors point to physical layer problems.
VSAN mismatch between switches
Why wrong: VSAN mismatch causes login failures, not CRC errors.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Faulty SFP or optical cable
CRC errors on a Fibre Channel link indicate physical-layer corruption of frames, typically caused by faulty optics, dirty or damaged fiber cables, or marginal signal integrity. Flapping occurs because the link repeatedly fails to maintain proper synchronization due to excessive bit errors, triggering port reset or re-initialization. A faulty SFP or optical cable directly introduces noise or attenuation that corrupts the data stream, leading to CRC errors and link instability.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
NPV mode enabled on the switch
Why it's wrong here
NPV mode does not cause CRC errors.
Incorrect zoning configuration
Why it's wrong here
Zoning affects access control, not physical errors.
Faulty SFP or optical cable
Why this is correct
CRC errors point to physical layer problems.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
VSAN mismatch between switches
Why it's wrong here
VSAN mismatch causes login failures, not CRC errors.
Common exam traps
Cisco often tests the distinction between physical-layer issues (CRC errors, flapping) and logical/configuration issues (VSAN mismatch, zoning, NPV), tempting candidates to select a configuration-based answer when the symptoms clearly point to a hardware fault.
Detailed technical explanation
CRC errors are computed by the receiving port using a 32-bit frame check sequence (FCS) per FC-FS-4; any mismatch indicates that the frame was altered during transmission. In Fibre Channel, link flapping due to CRC errors often triggers the 'link reset protocol' (LR/LRR) or causes the port to enter a 'recovery' state, which can be observed with 'show interface' counters showing 'CRC' and 'too many CRC' increments. Real-world scenarios include bent fiber cables, dirty connectors, or aging SFPs that degrade signal-to-noise ratio over time.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 350-601 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.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Storage Network — This question tests Storage Network — Read the scenario before looking for a memorised answer..
The correct answer is: Faulty SFP or optical cable — CRC errors on a Fibre Channel link indicate physical-layer corruption of frames, typically caused by faulty optics, dirty or damaged fiber cables, or marginal signal integrity. Flapping occurs because the link repeatedly fails to maintain proper synchronization due to excessive bit errors, triggering port reset or re-initialization. A faulty SFP or optical cable directly introduces noise or attenuation that corrupts the data stream, leading to CRC errors and link instability.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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