CCNA Network Infrastructure and Connectivity Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  SFP-10G-SR transceivers require single-mode fiber.

  Why it's wrong here

  SFP-10G-SR transceivers are designed for short-reach multimode fiber (MMF) links, not single-mode. They emit at 850 nm using a VCSEL, and rely on the large core of MMF (typically 50/125 µm OM3/OM4) to propagate the signal for 300–400 meters. Single-mode fiber has a 9 µm core that cannot efficiently couple the 850 nm VCSEL output, and the SR optical design does not support the connector or launch conditions needed for SMF.

• ✓

  SFP-10G-LR transceivers use 1310 nm wavelength and can reach up to 10 km over single-mode fiber.

  Why this is correct

  SFP-10G-LR transceivers implement 10GBASE-LR as defined by IEEE 802.3ae. They use a 1310 nm laser (typically a Fabry-Perot or DFB) with a narrow optical launch that is optimized for the 9 µm core of single-mode fiber (SMF). At 1310 nm, SMF has low attenuation and minimal chromatic dispersion, allowing the link to span up to 10 km, which is the standard reach for this optic.

• ✗

  SFP-10G-LR transceivers can only be used with multimode fiber.

  Why it's wrong here

  The SFP-10G-LR is specifically engineered for single-mode fiber; using multimode fiber would prevent it from reaching the rated 10 km distance. Multimode fiber has a larger core (50 or 62.5 µm) and exhibits differential mode delay and higher attenuation at 1310 nm, which destroys signal integrity over long spans. The LR optic's transmit power and receiver sensitivity are calibrated for SMF's low-loss characteristics, not for MMF's modal dispersion, so MMF is not a supported medium.

• ✓

  SFP-10G-SR transceivers typically use 850 nm wavelength over multimode fiber.

  Why this is correct

  SFP-10G-SR transceivers implement 10GBASE-SR and operate at 850 nm over multimode fiber (MMF). They use low-cost VCSELs and are designed for short data-center reaches: up to 300 meters over OM3 (50/125 µm laser-optimized) and 400 meters over OM4. The 850 nm wavelength pairs well with MMF because the larger core allows multiple modes to propagate, which is fine for the short distances SR is meant to cover.

• ✗

  SFP-10G-LR transceivers support distances up to 40 km over single-mode fiber.

  Why it's wrong here

  The 40 km distance is a feature of 10GBASE-ER/ER optics, not 10GBASE-LR. SFP-10G-LR transceivers are standardized for a maximum reach of 10 km over single-mode fiber at 1310 nm. The extended-reach 10G-ER optic uses 1550 nm with a higher-power laser and a more sensitive APD receiver to achieve 40 km; the LR optic's transmitter and receiver lack the optical budget for that longer span.

Option-by-option analysis

Why each answer is right or wrong

Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.

Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”

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