N10-009 Question 303
Single answerInterface issues: Increasing interface counters (Cyclic redundancy check (CRC), Runts, Giants, Drops)A network technician is troubleshooting poor performance on a file server connected to a switch access port. Users report slow transfers and intermittent failures. The switch shows the following for the server's interface over the last hour: CRC errors are steadily increasing, runts are incrementing, giants remain at 0, and output drops are not increasing. The cable was recently replaced, but the issue persists. Which of the following is the MOST likely cause of the problem?
- A
A duplex mismatch between the server NIC and the switch port
- B
An MTU mismatch causing oversized Ethernet frames
- C
A congested uplink causing packets to be dropped in the switch egress queue
- D
A broadcast storm from a looped access port
Show answer and explanation
Correct answer: A
Explanation
This question focuses on interpreting interface counters to identify the most likely operational issue. CRC errors indicate corrupted frames, commonly associated with physical-layer problems or duplex mismatches. Runts are frames smaller than the minimum Ethernet frame size and are also commonly seen with collisions or duplex mismatches, particularly when half-duplex behavior appears on one side of a link. Since giants are not increasing, oversized frames or jumbo frame/MTU problems are unlikely. Since output drops are not increasing, congestion on the switch egress queue is also unlikely. In real-world troubleshooting, CompTIA Network+ objectives expect candidates to correlate counters such as CRC, runts, giants, and drops with likely root causes. Vendor documentation from Cisco and other switch manufacturers consistently associates CRC errors with bad cabling, interference, or duplex problems, while runts are often linked to collisions and mismatched duplex settings. A practical best practice is to verify both sides of the link for auto-negotiation compatibility or explicitly matched speed/duplex settings after confirming cabling and transceiver health.
- A. Correct.
Correct. A duplex mismatch is a classic cause of increasing CRC errors and runts, especially when one side is operating at full duplex and the other at half duplex. Late collisions and frame corruption can result, which often appears as CRC errors and undersized frames on switch interfaces. Because the cable has already been replaced and giants are not increasing, a Layer 1 media fault is less likely than a negotiation or configuration mismatch on the link.
- B. Incorrect.
Incorrect. An MTU mismatch typically results in fragmentation issues at Layer 3 or oversized frames if jumbo frames are involved, which would be more consistent with increasing giant counters rather than runts. In this scenario, giants remain at 0, so oversized frames are not the primary symptom.
- C. Incorrect.
Incorrect. Egress queue congestion usually shows up as increasing output drops or discard counters, especially during bursts of traffic. The scenario specifically states that output drops are not increasing, which makes switch congestion a poor fit for the observed interface statistics.
- D. Incorrect.
Incorrect. A broadcast storm can cause severe congestion, high CPU utilization, and widespread packet loss, but it is not the most direct explanation for a combination of CRC errors and runts on a single server-facing interface. A storm would more commonly affect multiple ports and may increase drop counters due to congestion rather than specifically indicating frame corruption from a duplex mismatch.