220-1101 Question 323
Single answerDuplexA technician replaces an aging unmanaged switch with a newer managed switch in a small office. Soon after the change, one user's desktop can still reach network resources, but file copies are very slow and the NIC shows many late collisions and CRC errors. The desktop's network adapter is manually set to 100 Mbps full duplex, while the switch port is left at auto-negotiate. What is the most likely cause of the problem?
- A
A duplex mismatch between the PC and the switch port
- B
An IP address conflict caused by the new switch
- C
A bad DNS configuration on the desktop
- D
An MTU size mismatch between the desktop and the file server
Show answer and explanation
Correct answer: A
Explanation
The best answer is a duplex mismatch. In Ethernet environments, best practice is to leave both ends of a link set to auto-negotiate unless there is a specific reason to hard-code settings on both sides identically. A common real-world failure occurs when one side is forced to full duplex and the other remains on auto. Under IEEE 802.3 auto-negotiation behavior, the auto side may not negotiate duplex correctly and can default to half duplex, leading to collisions, CRC errors, and very poor throughput even though the link remains up. This is a standard troubleshooting point for A+ Core 1 networking: physical connectivity can appear normal while performance is degraded due to mismatched duplex settings. Vendor documentation from major switch and NIC manufacturers commonly recommends matching speed and duplex settings on both ends, with auto/auto preferred in modern networks.
- A. Correct.
Correct. This is a classic duplex mismatch scenario. When one side is manually forced to 100 Mbps full duplex and the other side is set to auto-negotiate, the auto-negotiating side may correctly detect speed but fall back to half duplex. That results in one side transmitting full duplex while the other expects half duplex, causing poor performance, collisions, and frame errors such as CRC errors. Slow file transfers with intermittent connectivity are common symptoms.
- B. Incorrect.
Incorrect. An IP address conflict typically causes intermittent or complete loss of connectivity, duplicate IP warnings, or ARP instability. It does not specifically explain late collisions and CRC errors on a single link after changing switch hardware.
- C. Incorrect.
Incorrect. DNS issues affect name resolution, such as being unable to browse to a hostname, but they do not create Layer 1/Layer 2 interface statistics like collisions or CRC errors. The user can still reach resources, which further suggests DNS is not the root issue.
- D. Incorrect.
Incorrect. An MTU mismatch can cause fragmentation problems or application-specific issues, especially with VPNs or certain protocols, but it does not typically produce late collisions on Ethernet links. The combination of manual full duplex on one side and auto-negotiate on the other is a stronger and more direct explanation.