N10-009 Question 304
Single answerInterface issues: Increasing interface counters (Cyclic redundancy check (CRC), Runts, Giants, Drops)A network administrator is troubleshooting poor application performance for users connected to a floor switch. On the switch uplink interface, the administrator notices that CRC errors and runts are steadily increasing, while utilization is moderate. The patch cable has already been replaced and the users still report intermittent slowdowns. Which action is the MOST appropriate next step to resolve the issue?
- A
Configure a larger MTU on the uplink to prevent undersized frames
- B
Investigate a duplex mismatch between the switch uplink and the connected device
- C
Enable port security so malformed frames are dropped before reaching the interface
- D
Replace the switch with a model that has larger packet buffers
Show answer and explanation
Correct answer: B
Explanation
CRC errors usually indicate frame corruption on the wire, often caused by physical-layer issues such as bad cabling, interference, failing transceivers, or a duplex mismatch. Runts are frames smaller than the minimum Ethernet frame size and are commonly associated with collisions or duplex problems, particularly when one side of a link is half duplex. In this scenario, the cable has already been replaced, making duplex mismatch the most likely next item to verify. By contrast, giants usually suggest oversized frames, often related to MTU/jumbo frame mismatches, and drops typically point to congestion, queue exhaustion, or buffer issues. Best practice is to check interface counters on both ends of the link, confirm speed and duplex settings match or are both set to auto-negotiation as appropriate, and review vendor guidance for Ethernet interface troubleshooting. These patterns are consistent with standard Ethernet troubleshooting recommendations published by major network vendors and reflected in CompTIA Network+ objectives on interface errors and performance analysis.
- A. Incorrect.
Incorrect. Increasing MTU affects the maximum frame size and is more relevant to giant frames or fragmentation-related issues. It does not prevent runts, which are undersized frames, and it does not address CRC errors caused by Layer 1/Layer 2 transmission problems. A common misconception is that any frame-size problem can be fixed with MTU changes, but runts and CRCs typically point elsewhere.
- B. Correct.
Correct. A duplex mismatch is a classic real-world cause of increasing CRC errors and runts, especially when utilization is not saturated and a cable replacement did not help. For example, one side operating at full duplex and the other at half duplex can produce late collisions, frame corruption, and undersized frames. Verifying speed/duplex settings or ensuring both sides negotiate correctly is the most appropriate next step.
- C. Incorrect.
Incorrect. Port security controls which MAC addresses are allowed on a port and can shut down or restrict a port due to policy violations. It does not correct CRC errors or runts caused by physical-layer faults or duplex mismatches. Someone might choose this option if they confuse malformed traffic with unauthorized traffic.
- D. Incorrect.
Incorrect. Larger packet buffers are more relevant to drops caused by congestion or microbursts, where frames are discarded because the interface cannot queue them fast enough. CRC errors and runts indicate corrupted or improperly transmitted frames, not a buffering limitation. Replacing the switch before checking duplex settings would be premature.