Question 185 of 464
N10-009 Network Troubleshooting Practice Question
A network administrator is troubleshooting an intermittent link between two switches connected by single-mode fiber. The interface log shows "Link up / Link down" events multiple times per hour. Which of the following is the most likely cause?
⚠ Common exam trap
The trap here is that candidates often jump to duplex mismatch or VLAN misconfiguration as common causes of link issues, but the intermittent 'Link up / Link down' pattern specifically points to a physical-layer problem like dirty connectors, not a Layer 2 configuration error.
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
✓
Dirty fiber connectors
Dirty fiber connectors cause intermittent signal loss by scattering or absorbing light, which leads to CRC errors and repeated link flaps as the optical transceiver struggles to maintain synchronization. This matches the 'Link up / Link down' pattern seen in the logs, especially on single-mode fiber where precise alignment is critical.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Incorrect VLAN configuration on the switch ports
Why it's wrong here
Incorrect VLAN configuration on switch ports affects logical connectivity and traffic forwarding at Layer 2, but it does not impact the physical link's ability to establish and remain stable. The physical link (Layer 1) between the two switches will stay up as long as the electrical or optical signals are correctly transmitted and received. VLAN mismatches typically manifest as traffic not reaching its destination, security violations, or spanning tree issues, rather than the physical port repeatedly losing its carrier signal.
When this WOULD be correct
A question where hosts in the same VLAN cannot communicate across switches, but the physical link is stable and up. The correct answer would be VLAN mismatch or misconfiguration on the trunk ports.
- ✗
Crossed fiber pairs
Why it's wrong here
Crossed fiber pairs, where the transmit (TX) fiber from one device is connected to the transmit (TX) port of the other, or receive (RX) to receive (RX), would fundamentally prevent the link from ever establishing. For a fiber link to come up, the light signal transmitted by one device must be received by the corresponding receive port on the other. With crossed pairs, neither device receives the expected light signal, so the link negotiation process cannot complete, resulting in a permanent 'down' state, not intermittent flapping.
When this WOULD be correct
This option would be correct in a scenario where two switches are connected via fiber but the link never establishes, or the interface shows 'not connected' despite both ends being up. For example, if a technician accidentally swaps the transmit and receive fibers, the link will not come up at all.
- ✓
Dirty fiber connectors
Why this is correct
Dirty fiber connectors are a very common cause of intermittent link flapping because microscopic dust or oil particles on the ferrule end-face can partially block or scatter the optical signal. This attenuation can cause the received light power to intermittently drop below the receiver's sensitivity threshold, leading to a loss of carrier and subsequent link re-negotiation. Cleaning the connectors with appropriate tools often restores stable link operation by ensuring clear signal transmission.
- ✗
Duplex mismatch between the switches
Why it's wrong here
A duplex mismatch between switches typically results in severe performance degradation, high error rates, and numerous collisions, but it does not usually cause the physical link to repeatedly go up and down. The physical layer (Layer 1) connection, which establishes the link state, remains stable because both devices are still sending and receiving electrical or optical signals. The mismatch primarily affects Layer 2 communication, leading to frame corruption and retransmissions rather than link instability.
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 N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓Dirty fiber connectorsCorrect answer▾
Why this is correct
Dirty fiber connectors are a very common cause of intermittent link flapping because microscopic dust or oil particles on the ferrule end-face can partially block or scatter the optical signal. This attenuation can cause the received light power to intermittently drop below the receiver's sensitivity threshold, leading to a loss of carrier and subsequent link re-negotiation. Cleaning the connectors with appropriate tools often restores stable link operation by ensuring clear signal transmission.
✗Incorrect VLAN configuration on the switch portsWrong answer — click to see why▾
Why this is wrong here
Incorrect VLAN configuration would cause connectivity issues for specific VLANs, not intermittent link flaps on the physical interface. The log shows physical layer events (link up/down), which are unrelated to VLAN settings.
★ When this WOULD be the correct answer
A question where hosts in the same VLAN cannot communicate across switches, but the physical link is stable and up. The correct answer would be VLAN mismatch or misconfiguration on the trunk ports.
Why candidates choose this
Candidates often associate any connectivity problem with VLAN misconfiguration, especially when switches are involved, without recognizing that link flaps are a physical layer symptom.
✗Crossed fiber pairsWrong answer — click to see why▾
Why this is wrong here
Crossed fiber pairs (e.g., TX/RX swapped) typically cause a complete link failure, not intermittent link flaps. The symptom described is frequent link up/down events, which is more consistent with physical layer issues like dirty connectors rather than a wiring error.
★ When this WOULD be the correct answer
This option would be correct in a scenario where two switches are connected via fiber but the link never establishes, or the interface shows 'not connected' despite both ends being up. For example, if a technician accidentally swaps the transmit and receive fibers, the link will not come up at all.
Why candidates choose this
Candidates may confuse intermittent link flaps with wiring issues because both involve physical layer problems. The term 'crossed' sounds like a plausible cause for unstable connectivity, leading them to overlook the more specific symptom of intermittent vs. persistent failure.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Last reviewed: Jun 11, 2026
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