220-1101 Networking Practice Question
A small business has a single router with four built-in LAN ports. They need to connect 12 workstations to the network. The technician adds a 24-port unmanaged switch and connects it to one of the router's LAN ports. After connecting all workstations, some users report intermittent network connectivity. The technician checks the switch and notices the collision LED is flashing rapidly. Which of the following is the MOST likely cause of the issue?
⚠ Common exam trap
A common mix-up: candidates assume a flashing collision LED indicates a faulty switch or a cable issue, but The 220-1101 exam specifically tests the concept that in a switched network, collisions should be rare, and a high collision rate almost always points to a duplex mismatch between the switch and another device.
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
✓
The network is experiencing excessive collisions due to a duplex mismatch
The rapid flashing of the collision LED on the switch indicates that the network is experiencing an abnormally high number of collisions. In a modern switched network, collisions should be virtually nonexistent because each switch port creates its own collision domain. The most common cause of excessive collisions in this scenario is a duplex mismatch, where one device (likely the router's LAN port) is operating at half-duplex while the switch port is set to auto-negotiate or is forced to full-duplex. Half-duplex requires collision detection (CSMA/CD), and when a full-duplex device transmits simultaneously, the half-duplex device interprets the resulting frame corruption as a collision, leading to repeated retransmissions and the observed LED behavior.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The switch is malfunctioning and needs to be replaced
Why it's wrong here
While a defective switch can produce a range of symptoms, hardware failure typically manifests as complete port failure, total link loss, or system-wide erratic behavior rather than a specific, persistent collision LED on one attached device. A failing switch would usually cause frames to be dropped silently or the port to stop negotiating a link altogether, not generate consistent collision indications. Since the router and switch are functioning at the link layer and the problem is isolated to collision counts, a duplex mismatch is a far more probable and testable cause before replacing hardware.
When this WOULD be correct
If the switch had failed to power on, showed no link lights on any ports, or caused all connected devices to lose connectivity entirely, then replacing the malfunctioning switch would be the correct answer.
- ✗
The router's LAN port is not configured for the correct speed
Why it's wrong here
A speed mismatch (e.g., 1000Mbps on one side and 100Mbps on the other) typically causes a link to not establish or to operate at the lower speed, but it does not cause collision LEDs to flash. Duplex is the likely culprit.
When this WOULD be correct
This option would be correct if the question described a scenario where a workstation or server connected to a switch port cannot establish a link or experiences intermittent connectivity, and the switch port shows errors like CRC errors or runts, indicating a speed mismatch between the device and the switch port.
- ✓
The network is experiencing excessive collisions due to a duplex mismatch
Why this is correct
A duplex mismatch occurs when one device operates at full-duplex while the other is forced to half-duplex. In half-duplex, the NIC cannot send and receive simultaneously, so it interprets any incoming frame during transmission as a collision, triggering the collision LED and causing frame retransmissions, CRC errors, and severely degraded throughput. This symptom—excessive collisions on a single switch port—classically points to mismatched duplex settings rather than a cabling or hardware fault.
- ✗
The switch is not receiving enough power from the router
Why it's wrong here
Standard unmanaged switches are powered by an external AC adapter, not by Power over Ethernet from the router; PoE is only relevant for devices like IP cameras or VoIP phones that explicitly draw power over the Ethernet cable. If the switch were underpowered, it would fail to boot, repeatedly reset, or drop all ports simultaneously—it would not selectively illuminate a collision LED while maintaining link on a specific port. Therefore, power delivery from the router is not physically involved in this scenario and cannot explain the observed collision indication.
When this WOULD be correct
A technician connects a PoE-powered device (e.g., an IP phone or PoE switch) to a router LAN port that does not support PoE, causing the device to power cycle or fail intermittently. The question would specify that the device requires PoE and the router lacks PoE capability.
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 220-1101 exam frequently reuses these exact scenarios with slightly different constraints.
✓The network is experiencing excessive collisions due to a duplex mismatchCorrect answer▾
Why this is correct
A duplex mismatch occurs when one device operates at full-duplex while the other is forced to half-duplex. In half-duplex, the NIC cannot send and receive simultaneously, so it interprets any incoming frame during transmission as a collision, triggering the collision LED and causing frame retransmissions, CRC errors, and severely degraded throughput. This symptom—excessive collisions on a single switch port—classically points to mismatched duplex settings rather than a cabling or hardware fault.
✗The switch is malfunctioning and needs to be replacedWrong answer — click to see why▾
Why this is wrong here
A malfunctioning switch would typically cause complete failure or port-specific issues, not a rapidly flashing collision LED on all ports. The collision LED indicates layer 1/2 issues like duplex mismatches, not hardware failure.
★ When this WOULD be the correct answer
If the switch had failed to power on, showed no link lights on any ports, or caused all connected devices to lose connectivity entirely, then replacing the malfunctioning switch would be the correct answer.
Why candidates choose this
Candidates may default to blaming hardware failure when they see an LED flashing, without understanding that collision LEDs specifically indicate duplex or cabling issues rather than a defective device.
✗The router's LAN port is not configured for the correct speedWrong answer — click to see why▾
Why this is wrong here
The router's LAN port speed configuration would not cause a rapidly flashing collision LED on the switch. Collision LEDs indicate physical-layer issues like duplex mismatches or excessive collisions, not speed mismatches, which typically cause link failures or errors but not collisions.
★ When this WOULD be the correct answer
This option would be correct if the question described a scenario where a workstation or server connected to a switch port cannot establish a link or experiences intermittent connectivity, and the switch port shows errors like CRC errors or runts, indicating a speed mismatch between the device and the switch port.
Why candidates choose this
Candidates may confuse speed and duplex settings, thinking that incorrect speed configuration can cause collisions, or they may overestimate the role of router configuration in a simple LAN setup with an unmanaged switch.
✗The switch is not receiving enough power from the routerWrong answer — click to see why▾
Why this is wrong here
Unmanaged switches are powered by an AC adapter, not PoE from the router's LAN port, so insufficient power from the router is not a plausible cause for intermittent connectivity or collision LED activity.
★ When this WOULD be the correct answer
A technician connects a PoE-powered device (e.g., an IP phone or PoE switch) to a router LAN port that does not support PoE, causing the device to power cycle or fail intermittently. The question would specify that the device requires PoE and the router lacks PoE capability.
Why candidates choose this
Candidates may confuse power-over-Ethernet (PoE) with standard Ethernet connectivity, assuming that a switch can draw power from a router's LAN port, especially when troubleshooting intermittent issues.
Analysis generated from the official 220-1101blueprint 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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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.