Troubleshooting Switch Interface Collisions
Short answer
D Duplex mismatch causes half-duplex interfaces to register simultaneous transmissions as collisions.
Question
Key Takeaways
Collisions on an interface indicate a duplex mismatch where one end is full-duplex and the other is half-duplex.
Identify duplex mismatch as the primary cause of interface collisions on newly installed switches.
Common mistake: Choosing 'Wrong speed' because speed mismatches also cause link issues, but they typically result in CRC errors or link flaps, not specifically collisions.
Expert Analysis
The key to this question lies in the specific symptom observed: collisions on a switch interface. In modern Ethernet, collisions are typically associated with half-duplex communication or physical layer issues.
- Duplex mismatch (D) is the correct answer. A duplex mismatch occurs when one device operates in full-duplex while the other operates in half-duplex. The half-duplex side listens for collisions before transmitting; if it detects data from the full-duplex side (which transmits without listening), it registers this as a collision. This results in high collision counts and performance degradation, exactly as described.
- Wrong speed (A) usually leads to link instability or no link at all, rather than sustained collisions. While speed mismatches can cause errors, they do not typically manifest primarily as collision counters increasing in the same predictable manner as duplex mismatches.
- Jumbo frames enabled (B) would likely result in frame size errors or dropped packets if MTU settings are inconsistent, but not collisions.
- Incorrect VLAN (C) would prevent connectivity entirely (no traffic flow) but would not generate collision statistics on the physical interface.
The comment correctly identifies that duplex mismatch causes the half-duplex side to detect simultaneous transmission/reception as collisions.
Official Reference
https://www.comp-tia.org/exams/network/n10-009
This link opens the official vendor documentation in a new tab.
Exam Strategy
When troubleshooting Layer 2 issues, always check the specific error type reported by the interface. 'Collisions' strongly point to duplex mismatches or half-duplex legacy configurations. 'CRC errors' often suggest cabling or speed issues. Use these specific symptoms to narrow down candidates quickly.
Frequently Asked Questions
Why do collisions happen in full-duplex mode?
How does a duplex mismatch affect performance?
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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ERB14 👍 1 Answered DDuplex mismatch: A duplex mismatch occurs when one end of an Ethernet link is set to full-duplex and the other end is set to half-duplex. The side configured for half-duplex detects traffic being sent and received simultaneously as a collision, leading to high collision counters on the interface.
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