N10-009 Question 300
Single answer5.2 Given a scenario, troubleshoot common cabling and physical interface issues.A network technician installs a new Cat 6 horizontal cable run from a patch panel in the IDF to a user's wall jack. After patching the link to a 1 Gbps switch port and connecting a workstation, the workstation only negotiates at 100 Mbps. The switch port shows no errors, and replacing the patch cords on both ends does not change the result. A cable tester reports continuity on all eight conductors, but the pair map shows pins 1 and 2 are on one twisted pair while pins 3 and 6 are split across conductors from different pairs. Which issue is the MOST likely cause of the reduced link speed?
- A
The horizontal cable exceeds the maximum Ethernet distance and should be shortened
- B
The cable was terminated with a split pair, causing signal integrity problems
- C
The link requires a crossover cable instead of straight-through wiring
- D
The switch port is failing to provide sufficient PoE to the workstation
Show answer and explanation
Correct answer: B
Explanation
The best answer is that the cable was terminated with a split pair. This is a classic physical-layer issue in structured cabling: continuity may appear correct because each pin connects end-to-end, but the conductors are not maintained as proper twisted pairs. Ethernet signaling over copper relies on the twists in each pair to reduce electromagnetic interference and crosstalk. When pairs are split, the link may still come up, but performance often drops and gigabit negotiation frequently fails, resulting in a 100 Mbps connection. In practice, this is why technicians should use a cable certifier or tester that validates pair mapping, not just continuity. Best practices from TIA-568 structured cabling standards and IEEE 802.3 Ethernet requirements emphasize correct pair termination and maintaining pair twists as close as possible to the termination point.
- A. Incorrect.
Incorrect. Excessive cable length can cause attenuation and link instability, but the scenario does not indicate a run beyond Ethernet limits. Also, the tester specifically identifies a pair-mapping problem, which is a stronger indicator of the root cause than distance. For twisted-pair Ethernet, the permanent link and channel length limits are defined in TIA/EIA structured cabling standards and IEEE Ethernet guidance, but nothing here suggests that limit has been exceeded.
- B. Correct.
Correct. A split pair occurs when individual conductors are placed on the correct pins for continuity but are not kept in their proper twisted pairs. Ethernet over twisted pair depends on the correct pairings to control crosstalk and maintain signal quality. This commonly allows a basic continuity test to pass while causing degraded performance, negotiation problems, or fallback to 100 Mbps instead of 1000 Mbps. Gigabit Ethernet requires all four pairs to be correctly terminated.
- C. Incorrect.
Incorrect. Modern switch ports and NICs support auto-MDI/MDIX in most environments, so a crossover cable is usually not required. More importantly, the cable tester result points to incorrect pairing inside the termination, not an incorrect cable type between devices. Straight-through versus crossover would not explain a split-pair reading on the installed horizontal cable.
- D. Incorrect.
Incorrect. PoE is unrelated to Ethernet speed negotiation for a standard workstation, which typically does not use PoE at all. Insufficient PoE could affect powered devices such as IP phones, cameras, or access points, but it would not cause the cable tester to report split pairs. This option reflects a common misconception of confusing power delivery issues with Layer 1 cabling faults.