CCNA Network Infrastructure and Connectivity Practice Question
A network engineer notices that a switch port connected to a legacy server is experiencing late collisions and the server reports excessive retransmissions. The switch port is configured for auto-negotiation and shows a negotiated speed of 100 Mbps and duplex full. The server's NIC is manually set to 100 Mbps and half-duplex. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the concept that auto-negotiation mismatches (e.g., one side set to manual) cause duplex mismatches, and candidates mistakenly think the issue is speed mismatch or that both sides must be manually set, but the trap here is that the server's manual half-duplex setting overrides the auto-negotiation result, creating a duplex mismatch that produces late collisions.
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
✓
A duplex mismatch exists between the switch port and the server NIC.
The switch port is auto-negotiating to full-duplex while the server's NIC is manually set to half-duplex. This creates a duplex mismatch: the switch transmits expecting no collisions (full-duplex), but the server, operating in half-duplex, detects collisions when the switch sends frames while the server is transmitting. Late collisions occur because the collision is detected after the first 64 bytes of the frame, and the server's half-duplex CSMA/CD logic forces retransmissions, matching the symptoms described.
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 port is incorrectly configured for auto-negotiation and should be manually set to match the server's NIC.
Why it's wrong here
Manually setting the switch port to 100 Mbps/half-duplex would resolve the mismatch, but the root cause is the mismatch itself, not the auto-negotiation. The problem is the duplex mismatch, and disabling auto-negotiation is merely a workaround.
- ✗
The server's NIC is failing, causing cyclic redundancy check (CRC) errors and forcing retransmissions.
Why it's wrong here
A failing NIC or bad cabling typically produces FCS/CRC errors, runts, or jabber at the physical layer, not late collisions. Late collisions occur after the 64-byte slot time when a half-duplex device fails to sense that a full-duplex peer is already transmitting, which is the classic signature of a duplex mismatch. CRC errors and retransmissions can be secondary consequences of that mismatch, but the underlying fault is the speed/duplex negotiation, not a defective network interface card.
- ✓
A duplex mismatch exists between the switch port and the server NIC.
Why this is correct
The switch port negotiated full-duplex at 100 Mbps (as shown in the switch output), while the server NIC is hard-coded to half-duplex. This mismatch causes exactly the observed symptoms: late collisions on the full-duplex switch port and excessive retransmissions on the half-duplex server.
- ✗
The switch port is overloaded by a broadcast storm, causing an excessive number of collisions.
Why it's wrong here
Broadcast storms generate a high volume of broadcast traffic, which can saturate the link and cause slow performance, but they do not cause late collisions. Late collisions are a layer-1 timing issue, not a traffic-load issue.
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓A duplex mismatch exists between the switch port and the server NIC.Correct answer▾
Why this is correct
The switch port negotiated full-duplex at 100 Mbps (as shown in the switch output), while the server NIC is hard-coded to half-duplex. This mismatch causes exactly the observed symptoms: late collisions on the full-duplex switch port and excessive retransmissions on the half-duplex server.
✗The switch port is incorrectly configured for auto-negotiation and should be manually set to match the server's NIC.Wrong answer — click to see why▾
Why this is wrong here
Misunderstanding that auto-negotiation always causes duplex mismatches, when in fact a mismatch occurs because one side is manually configured while the other uses auto-negotiation to negotiate an incompatible mode.
✗The server's NIC is failing, causing cyclic redundancy check (CRC) errors and forcing retransmissions.Wrong answer — click to see why▾
Why this is wrong here
Confusing CRC errors with late collisions. Late collisions are a layer-1 timing issue, not a data integrity problem.
✗The switch port is overloaded by a broadcast storm, causing an excessive number of collisions.Wrong answer — click to see why▾
Why this is wrong here
Attributing all network performance problems to broadcast storms, ignoring the specific error counter 'late collisions' that points directly to a duplex mismatch.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Go deeper
Related to this question
Learn chapter
Collision Domains and Broadcast Domains
Key term
Switch
A switch is a networking device that connects devices on a local area network and uses MAC addresses to forward data only to the intended recipient.
Key term
Frame
A frame is a unit of data at the Data Link layer of networking, containing header, payload, and trailer information for local network delivery.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco 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 200-301 exam.