Quick Answer
The answer is a faulty SFP or cable. When a UCS domain experiences intermittent HBA link down despite correct SAN zoning and VSAN configuration, the issue almost always resides at the physical layer, where a marginal SFP transceiver or damaged fiber/copper cable introduces sporadic signal loss. This causes the host interface to flap between link up and link down states, a classic symptom that cannot be resolved by logical configuration changes. On the Cisco DCCOR 350-601 exam, this scenario tests your ability to isolate storage connectivity problems by ruling out zoning and VSAN misconfigurations first; a common trap is to blame the fabric interconnect or HBA driver when the root cause is a cheap, failing optic. Remember the memory tip: “Zoning and VSANs are right, so check the light”—always inspect the physical optics and cables before diving into software troubleshooting.
This 350-601 practice question tests your understanding of compute. 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.
A UCS domain is experiencing intermittent storage connectivity. The storage admin confirms the SAN is properly zoned and the VSANs are configured. The UCS admin finds that the host interface (HBAs) are showing 'Link Down' intermittently. Which of the following is a likely cause?
Incompatible HBA firmware
Why wrong: Firmware incompatibility typically causes persistent errors or no link.
Mismatched fabric failover policy
Why wrong: Failover policy affects path selection, not physical link state.
Incorrect Fibre Channel interface mode on the FI
Why wrong: Mode mismatch usually causes persistent link failure, not intermittent.
Faulty SFP or cable
Physical layer issues often cause intermittent link flapping.
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 cable
Intermittent 'Link Down' on HBAs, despite correct SAN zoning and VSAN configuration, strongly points to a physical-layer issue. Faulty SFPs or damaged cables cause intermittent link flaps, which manifest as sporadic HBA link-down events. This is the most common cause of such symptoms in UCS environments.
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.
Incompatible HBA firmware
Why it's wrong here
Firmware incompatibility typically causes persistent errors or no link.
Mismatched fabric failover policy
Why it's wrong here
Failover policy affects path selection, not physical link state.
Incorrect Fibre Channel interface mode on the FI
Why it's wrong here
Mode mismatch usually causes persistent link failure, not intermittent.
Faulty SFP or cable
Why this is correct
Physical layer issues often cause intermittent link flapping.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Cisco often tests the distinction between physical-layer issues (SFP/cable) and configuration or policy errors, tempting candidates to overthink with fabric failover or interface mode when the symptom is a simple link flap.
Detailed technical explanation
Fibre Channel link establishment relies on the SFP's optical transceiver and cable integrity; a degraded SFP can cause CRC errors, signal loss, and link resets. In UCS, the 'show interface fc' command on the FI can reveal incrementing 'Link Failure' or 'Sync Loss' counters, which help isolate faulty SFPs. Real-world scenarios often involve bent fiber cables or dirty connectors causing intermittent link drops that are difficult to reproduce.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Compute — This question tests Compute — Read the scenario before looking for a memorised answer..
The correct answer is: Faulty SFP or cable — Intermittent 'Link Down' on HBAs, despite correct SAN zoning and VSAN configuration, strongly points to a physical-layer issue. Faulty SFPs or damaged cables cause intermittent link flaps, which manifest as sporadic HBA link-down events. This is the most common cause of such symptoms in UCS environments.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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