N10-009 Question 317
Single answer5.4 Given a scenario, troubleshoot common performance issues.A network administrator receives complaints that users in one office experience very slow file transfers to a local file server during business hours. The server is connected to a switch at 1 Gbps full duplex, and users can still reach the server, but transfers that normally take seconds are taking several minutes. The administrator checks the switch port connected to the server and sees a rapidly increasing number of late collisions and FCS errors. Which of the following is the MOST likely cause of the performance issue?
- A
A duplex mismatch between the server NIC and the switch port
- B
A routing loop caused by a misconfigured default gateway on the server
- C
A DNS resolution failure causing clients to retry file transfer requests
- D
Insufficient DHCP scope space causing clients to use APIPA addresses
Show answer and explanation
Correct answer: A
Explanation
The key troubleshooting clue is the switch port statistics: late collisions and FCS errors are commonly associated with duplex mismatches or other physical/link-layer issues. In modern switched Ethernet networks, collisions should not normally occur on properly configured full-duplex links. A mismatch causes one side to transmit as if collisions are impossible while the other side still uses CSMA/CD behavior, resulting in poor performance, frame corruption, and retransmissions. This aligns with standard Ethernet troubleshooting practices documented by major network vendors such as Cisco and in general NIC/switch best practices: verify speed/duplex settings, prefer auto-negotiation on both ends when supported, and check interface counters for incrementing errors. The most likely root cause is therefore a duplex mismatch between the server NIC and the switch port.
- A. Correct.
Correct. Late collisions and FCS errors on an Ethernet link strongly suggest a duplex mismatch, such as one side operating at full duplex and the other at half duplex. This creates excessive retransmissions, poor throughput, and intermittent performance problems while basic connectivity may still appear to work. In real environments, this is a classic cause of slow transfers and high interface error counters.
- B. Incorrect.
Incorrect. A default gateway issue would primarily affect traffic destined for remote networks, not local traffic between users and a local file server on the same switched network. It also would not typically produce late collisions and FCS errors on the switch port connected to the server.
- C. Incorrect.
Incorrect. DNS problems can delay name resolution or cause connection failures if clients cannot resolve the server name, but they do not generate Layer 1/Layer 2 interface errors like late collisions and FCS errors. Since users can still access the server and the issue is degraded transfer speed, DNS is not the best explanation.
- D. Incorrect.
Incorrect. DHCP scope exhaustion can cause some clients to self-assign APIPA addresses and lose normal network connectivity, but that would present as inability to communicate on the production subnet rather than slow file transfers for already-connected users. It also would not explain interface-level collision and frame check sequence errors on the server's switch port.