N10-009 Question 299
Single answer5.2 Given a scenario, troubleshoot common cabling and physical interface issues.A technician installs a new Cat 6 drop for a user's workstation. After connecting the PC to a switch port, the link LED on the switch turns on, but the workstation can only connect at 100 Mbps instead of 1 Gbps. The switch port is configured for auto-negotiation, and the NIC supports 1 Gbps. Replacing the patch cable at the desk does not change the result. Which of the following is the MOST likely cause of the problem?
- A
The horizontal cable has an open or miswired pair, preventing all four pairs from being used
- B
The switch port is suffering from a duplex mismatch caused by auto-negotiation being enabled
- C
The cable run exceeds the PoE power budget for the switch port
- D
The workstation is connected with a rollover cable instead of a straight-through Ethernet cable
Show answer and explanation
Correct answer: A
Explanation
The best answer is that the installed cabling has an open or miswired pair. In real-world troubleshooting, a link that comes up at 100 Mbps but not 1 Gbps strongly suggests a cabling fault affecting one or more pairs. 100BASE-TX uses two pairs, while 1000BASE-T uses all four pairs. A cable tester or certifier would typically reveal an open pair, split pair, reversed pair, or poor termination at the patch panel or jack. Best practices from structured cabling standards such as TIA/EIA-568 emphasize correct pair termination, maintaining twist as close as possible to the punchdown point, and validating the run after installation. A practical next step would be to test the permanent link with a cable tester and inspect both terminations for T568A/T568B consistency and punchdown quality.
- A. Correct.
Correct. 1000BASE-T requires all four twisted pairs to be correctly terminated and functional. If one pair is open, split, or otherwise miswired in the horizontal run, the link may still come up at 10/100 Mbps because those standards can operate with fewer pairs, but Gigabit Ethernet will fail to negotiate properly. This is a classic symptom of a bad punchdown, damaged cable, or improper termination in structured cabling.
- B. Incorrect.
Incorrect. A duplex mismatch typically causes poor performance, late collisions, CRC errors, and retransmissions, but it does not usually force a Gigabit-capable link down to 100 Mbps simply because auto-negotiation is enabled. In fact, 1000BASE-T normally relies on auto-negotiation to establish Gigabit operation.
- C. Incorrect.
Incorrect. PoE power budget issues affect whether powered devices such as IP phones, cameras, or access points receive power. They do not cause a standard workstation Ethernet link to negotiate from 1 Gbps down to 100 Mbps. This option confuses power delivery with physical-layer pair requirements.
- D. Incorrect.
Incorrect. A rollover cable is used for serial console access, not for connecting a workstation NIC to an Ethernet switch port. If a rollover cable were used, the connection would not behave like a normal Ethernet link. The scenario also indicates the patch cable was replaced without resolving the issue, pointing more strongly to the permanent link or termination.