N10-009 Question 96
Single answerA network administrator is upgrading a data center row by adding a new top-of-rack switch. The switch must connect to four nearby servers at 10 GbE over distances of less than 5 meters and to an aggregation switch located 450 meters away in another part of the building. The fiber route to the aggregation switch passes through the building's air-handling space above the ceiling. The company wants a standards-based, cost-conscious design that meets code requirements and avoids unnecessary transceivers where possible. Which solution BEST meets these requirements?
- A
Use SFP+ direct attach copper (DAC) cables for the server links, and use single-mode fiber with plenum-rated jacketed cable and appropriate 10 GbE optics for the 450-meter uplink
- B
Use multimode fiber for all links, including the server connections and the 450-meter uplink, because multimode supports 10 GbE and is less expensive than other media
- C
Use coaxial cable for the server links because it is inexpensive and supports high throughput, and use non-plenum single-mode fiber for the uplink to reduce installation cost
- D
Use Cat 5e for the server links at 10 GbE and direct attach copper (DAC) for the 450-meter uplink because DAC avoids the need for optical modules
Show answer and explanation
Correct answer: A
Explanation
The best answer is to use DAC for the short 10 GbE server connections and single-mode fiber for the 450-meter uplink, with plenum-rated cable because the run passes through an air-handling space. In practice, SFP+ DAC, also called twinax, is widely used for short, inexpensive 10 GbE connections in racks because it avoids separate optical transceivers. For longer distances, fiber is preferred. At 10 GbE, single-mode fiber with the proper optics is the most reliable standards-based choice for a 450-meter building uplink. Multimode can support 10 GbE, but distance depends on the Ethernet optic and fiber type; common 10GBASE-SR deployments do not make 450 meters a universally safe assumption. Plenum-rated cable is used in plenum spaces to comply with fire and building safety requirements. Relevant best-practice references include IEEE 802.3 Ethernet physical layer standards for 10 GbE media types and distances, along with NEC and local building code guidance for plenum versus non-plenum cable installations.
- A. Correct.
Correct. For very short 10 GbE connections inside a rack or between adjacent racks, SFP+ DAC/twinax is a common and cost-effective choice because it integrates the transceivers into the cable assembly and typically supports short distances such as 1-7 meters depending on the cable type. For the 450-meter uplink, single-mode fiber is the best fit because it supports long-distance links with the proper 10 GbE optics and avoids the distance constraints that can affect multimode at 10 GbE. Because the cable passes through an air-handling space, plenum-rated cable is the correct code-conscious choice for the jacket. This option aligns with real-world data center practice and Ethernet standards usage.
- B. Incorrect.
Incorrect. Multimode fiber can support 10 GbE, but using it for the short server links is not the most cost-conscious approach because it would require optical transceivers on both ends instead of simpler DAC assemblies. More importantly, a 450-meter 10 GbE uplink over multimode is not a safe general choice without very specific optics and fiber grades. Common 10GBASE-SR multimode implementations have shorter distance limits than 450 meters, depending on OM rating. This option reflects the common misconception that multimode is automatically suitable for all in-building fiber runs at 10 GbE just because it is cheaper.
- C. Incorrect.
Incorrect. Coaxial cable is not the appropriate medium for modern 10 GbE server-to-switch connections in this scenario. While coax has legitimate uses, such as broadband and some legacy Ethernet implementations, it is not the normal choice for current data center 10 GbE switch uplinks or server access links. The non-plenum part is also wrong because the path is through an air-handling space, where plenum-rated cable is typically required by building code to limit smoke and toxic fumes. This option combines two realistic misconceptions: overgeneralizing coaxial cable capability and ignoring plenum requirements.
- D. Incorrect.
Incorrect. Cat 5e is not the best choice for reliable 10 GbE server links. 10GBASE-T generally requires cabling designed for 10 GbE performance, with Cat 6a being the common recommendation for full 100-meter support. In addition, DAC is not suitable for a 450-meter uplink; DAC/twinax is intended for very short runs, not building-length backbone links. This option targets two common errors: assuming any twisted-pair cable can carry 10 GbE effectively and confusing short-range DAC with long-distance backbone media.