220-1101 Networking Practice Question
A network technician is installing a new fiber optic link between two switches located in different buildings. The technician uses identical SFP modules on both ends and connects them with a single fiber patch cable. After powering on, the link lights on both switches remain off. The technician verifies the SFP modules are compatible with the switches and that the cable is not damaged. Which of the following should the technician check NEXT?
⚠ Common exam trap
A common mix-up: candidates assume a single fiber cable is a simplex cable (one strand) and forget that duplex fiber requires a transmit/receive crossover, leading them to incorrectly blame software settings like VLANs or duplex mode.
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
✓
Swap the transmit and receive connections at one end of the fiber cable
Fiber optic communication requires two separate strands: one for transmitting (TX) and one for receiving (RX). If a single fiber patch cable is used, it typically contains two fibers, but the connectors must be crossed at one end so that the TX of one switch connects to the RX of the other. Since the link lights are off despite compatible SFPs and an undamaged cable, the most likely issue is a polarity mismatch, where both ends are wired straight through (TX to TX, RX to RX), preventing the optical signal from being received.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Swap the transmit and receive connections at one end of the fiber cable
Why this is correct
A duplex fiber link uses two strands: one carries the transmit signal and the other receives it, so the TX laser on each switch must connect to the RX receiver at the far end. If a strand is accidentally terminated with TX-to-TX or RX-to-RX polarity, the link light remains dark because neither side ever detects incoming light. Swapping the paired connectors at only one end reverses the roles of the two strands, matching every TX to an RX and bringing the physical link up without changing anything on the opposite end.
- ✗
Change the duplex settings on both switches to half-duplex
Why it's wrong here
Fiber Ethernet links are engineered for full-duplex operation, using separate fibers for transmit and receive to send and receive simultaneously with no collisions. Forcing both switches to half-duplex does not reverse the physical signal paths and will not make a reversed TX/RX pair function, and if only one switch is changed, auto-negotiation can fail entirely. It also caps throughput and disables collision-free operation, so this setting is never a valid fix for a missing link light.
- ✗
Update the firmware on both switches
Why it's wrong here
Firmware controls switch management, routing, port configuration, and bug fixes, but a dead link light is a layer-1 indicator: no physical signal is being received by the SFP or transceiver. Updating the firmware would not alter cabling polarity, SFP laser power, or improper connector seating, so the transmit-and-receive mismatch would persist after the upgrade. While a firmware bug might affect port advertisement in rare cases, the correct immediate action is to verify physical connectivity and polarity rather than reboot the device via a firmware update.
- ✗
Verify that the VLAN configuration matches on both ends
Why it's wrong here
VLAN configuration operates at layer 2 and determines which logical broadcast domains are carried over a trunk after the circuits are up. A VLAN mismatch or an untagged/access port mismatch can drop or misdirect traffic but does not stop the physical link from establishing: the switch will still show a link LED because an optical signal is detected. Troubleshooting a dark link requires checking the physical layer first, so inspect SFP lasers, fiber continuity, and polarity before considering VLAN pruning or allowed lists.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1101 practice question is part of Courseiva's free CompTIA 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 220-1101 exam.