N10-009 Question 97
Single answerA network administrator is upgrading links in a data center. The new top-of-rack switch has several SFP+ ports, and a nearby storage array in the same rack also has SFP+ ports. The administrator needs a cost-effective 10 Gb connection between the two devices over a distance of 3 meters. At the same time, the administrator must install new cabling above a dropped ceiling that is part of the building's air-handling space. Which solution best meets both requirements?
- A
Use a DAC/twinax cable for the 3-meter switch-to-storage link and plenum-rated cable above the dropped ceiling
- B
Use single-mode fiber with LR optics for the 3-meter switch-to-storage link and non-plenum cable above the dropped ceiling
- C
Use RG-6 coaxial cable for the 10 Gb switch-to-storage link and plenum-rated cable above the dropped ceiling
- D
Use multimode fiber with SC connectors only because DAC is limited to 1 Gb, and use non-plenum cable above the dropped ceiling
Show answer and explanation
Correct answer: A
Explanation
The best answer is to use a DAC/twinax cable for the 3-meter SFP+ connection and plenum-rated cable in the air-handling ceiling space. In real deployments, DAC is a standard short-distance option for 10 GbE and other high-speed links because it is inexpensive, simple, and designed specifically for interfaces such as SFP+. Fiber is more appropriate when distance, EMI concerns, or patch-panel design justify it, but for a few meters in the same rack, DAC is typically preferred. Plenum-rated cable is required in plenum spaces due to fire safety considerations; using non-plenum cable there violates best practices and often code requirements. These decisions align with common Ethernet deployment practices under IEEE 802.3 implementations for 10 GbE physical media and with building safety standards such as NEC guidance for plenum spaces.
- A. Correct.
Correct. Direct attach copper (DAC), also called twinax, is commonly used for very short high-speed connections such as 10 GbE SFP+ links within the same rack or between adjacent racks. At 3 meters, DAC is a practical and cost-effective choice because it avoids the cost of separate transceivers and fiber patching. For cabling installed in plenum spaces, such as many air-return areas above dropped ceilings, plenum-rated cable is required by building and fire codes because it produces less smoke and has more fire-resistant jacket materials.
- B. Incorrect.
Incorrect. Single-mode fiber with 10GBASE-LR optics can support long distances, typically up to 10 km, but it is not the most cost-effective solution for a 3-meter in-rack link when both devices have SFP+ ports that can use DAC. The second part is also wrong: non-plenum cable should not be used in a plenum space. This option reflects the misconception that higher-performance or longer-distance media is automatically the best technical choice regardless of cost and environment.
- C. Incorrect.
Incorrect. RG-6 coaxial cable is not used as a standard medium for 10 GbE links between SFP+ switch and storage ports. SFP+ interfaces are designed for DAC or fiber modules, not traditional coax Ethernet in this scenario. Although plenum-rated cabling is correct for the ceiling space, the primary link medium is wrong. This distractor targets confusion between legacy or broadband coax use and modern Ethernet uplink media.
- D. Incorrect.
Incorrect. Multimode fiber can certainly support 10 GbE, but the claim that DAC is limited to 1 Gb is false. DAC is widely used for 10 GbE and higher speeds in short-reach data center connections. Also, requiring SC connectors only is inaccurate because 10 Gb multimode optics commonly use LC connectors with SFP+ modules. Finally, non-plenum cable is inappropriate for plenum spaces. This option combines several common misconceptions about connector types, media capabilities, and fire-code requirements.