Quick Answer
The answer is that the interface LED is disabled in software configuration via the `no led` command. This is the most likely cause when a Cisco MDS interface LED is off even though the interface shows up/up, because the LED state is independent of the link state—the hardware can be fully operational and passing traffic while the LED is administratively turned off to reduce visual noise in a dense data center. On the Cisco DCCOR / CCNP Data Center Core 350-601 exam, this scenario tests your understanding of the distinction between physical link status and administrative LED control, often appearing as a trap where candidates mistakenly suspect a hardware fault or a transceiver issue. A common memory tip is: “LED off, link up—check the config, not the cup,” reminding you that the software toggle is the first place to look rather than assuming a physical failure.
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 the LED of an FC interface is off on a Cisco MDS switch. The interface is up/up. 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.
The port LED firmware is corrupted.
Why wrong: This is unlikely; LED behavior is simple.
The interface is administratively down.
Why wrong: The interface is up/up, so it is not admin down.
The link is up but the LED is disabled in software configuration.
On some MDS switches, the LED can be turned off via 'no led enable' interface command.
The SFP is faulty.
Why wrong: A faulty SFP would cause the interface to be down.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The link is up but the LED is disabled in software configuration.
On Cisco MDS switches, the interface LED can be administratively disabled via the `no led` command in interface configuration mode, even when the interface is fully operational (up/up). This allows operators to selectively turn off LEDs for troubleshooting or to reduce visual noise in a data center, without affecting traffic. The LED being off while the interface shows up/up directly points to this software-based LED disablement.
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.
The port LED firmware is corrupted.
Why it's wrong here
This is unlikely; LED behavior is simple.
The interface is administratively down.
Why it's wrong here
The interface is up/up, so it is not admin down.
The link is up but the LED is disabled in software configuration.
Why this is correct
On some MDS switches, the LED can be turned off via 'no led enable' interface command.
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.
The SFP is faulty.
Why it's wrong here
A faulty SFP would cause the interface to be down.
Common exam traps
Cisco often tests the misconception that an LED off always indicates a hardware or link issue, when in fact the LED can be software-disabled independently of the interface operational state.
Detailed technical explanation
The `no led` command under interface configuration mode disables the port LED by clearing the LED_ENABLE bit in the port's hardware register, while the port itself remains fully operational. This is often used in large deployments to avoid confusion when multiple ports are intentionally dark (e.g., for future use) or to comply with data center lighting policies. The show interface command will still report the interface as up/up, confirming that the LED state is independent of the link state.
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.
Storage Network — study guide chapter
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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: The link is up but the LED is disabled in software configuration. — On Cisco MDS switches, the interface LED can be administratively disabled via the `no led` command in interface configuration mode, even when the interface is fully operational (up/up). This allows operators to selectively turn off LEDs for troubleshooting or to reduce visual noise in a data center, without affecting traffic. The LED being off while the interface shows up/up directly points to this software-based LED disablement.
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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