CCNA Network Infrastructure and Connectivity Practice Question
Which TWO statements about SFP transceivers and fiber optic cabling are correct?
⚠ Common exam trap
Cisco often tests the misconception that multimode fiber always uses lasers for long distances, when in fact multimode fiber is optimized for shorter runs with LED or VCSEL sources, while single-mode fiber uses lasers for extended reach.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
1000BASE-SX SFP transceivers are designed for multimode fiber and typically support distances up to 550 meters.
Option B is correct because 1000BASE-SX is the IEEE 802.3z standard for Gigabit Ethernet over multimode fiber (MMF), using short-wavelength 850 nm VCSEL/laser transmitters and supporting distances up to 550 m with 50-micron MMF (and 220–275 m with 62.5-micron MMF). Option D is correct because 1000BASE-LX uses long-wavelength 1310 nm optics and, when paired with single-mode fiber (SMF), can reach approximately 5 km (and up to 10 km in some implementations), making it suitable for campus backbone runs. Option A is wrong because SFP (Small Form-factor Pluggable) transceivers are hot-swappable modular devices that plug into SFP slots or cages, not permanently soldered to the motherboard. Option C is wrong because multimode fiber is typically used with LED or VCSEL sources for shorter distances; long-distance transmission is the domain of single-mode fiber with laser transmitters. Option E is wrong because single-mode fiber has a much smaller core diameter (about 9 microns) than multimode fiber (50 or 62.5 microns), which is precisely what enables single-mode's lower modal dispersion and longer reach.
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 transceivers are permanently installed on the switch motherboard and cannot be replaced.
Why it's wrong here
SFP transceivers are hot-swappable modules inserted into SFP slots, so they can be replaced or changed to match the required media type. Fixed, soldered optics describe older GBIC-style or onboard uplink designs. Replaceability is exactly why SFPs are chosen for flexible uplink provisioning.
- ✓
1000BASE-SX SFP transceivers are designed for multimode fiber and typically support distances up to 550 meters.
Why this is correct
1000BASE-SX operates at 850 nm over multimode fibre, supporting up to 550 m on 50-micron OM2 cabling. This satisfies the stem's requirement for a correct statement pairing the SX transceiver with multimode media and its typical distance limit.
- ✗
Multimode fiber typically uses laser-based transmitters for long-distance transmission.
Why it's wrong here
Multimode fibre uses LED or VCSEL transmitters for short-reach, high-speed links within buildings; lasers driving single-mode are used for long-distance transmission. Multimode's modal dispersion limits distance. It would be the correct choice for short campus runs where cheaper optics suffice.
- ✓
Single-mode fiber (SMF) with 1000BASE-LX SFP transceivers can support distances up to 5 km.
Why this is correct
1000BASE-LX SFP transceivers are designed for single-mode fiber and support a maximum distance of 5 km according to IEEE 802.3z. Vendor-specific 'LX/LH' transceivers may extend this to 10 km, but the CCNA exam focuses on standard specifications.
- ✗
Single-mode fiber has a larger core diameter than multimode fiber, allowing longer distances.
Why it's wrong here
Single-mode fibre has a much smaller core (around 9 microns) than multimode (50 or 62.5 microns), which is precisely why it supports longer distances with less modal dispersion. Multimode's larger core suits shorter runs. Single-mode is correct for long-haul or campus backbone links.
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.
✓1000BASE-SX SFP transceivers are designed for multimode fiber and typically support distances up to 550 meters.Correct answer▾
Why this is correct
1000BASE-SX operates at 850 nm over multimode fibre, supporting up to 550 m on 50-micron OM2 cabling. This satisfies the stem's requirement for a correct statement pairing the SX transceiver with multimode media and its typical distance limit.
✗SFP transceivers are permanently installed on the switch motherboard and cannot be replaced.Wrong answer — click to see why▾
Why this is wrong here
This statement is false because SFP slots are designed for modular, replaceable transceivers.
✗Multimode fiber typically uses laser-based transmitters for long-distance transmission.Wrong answer — click to see why▾
Why this is wrong here
Laser transmitters are used in single-mode fiber for longer distances; multimode uses LED/VCSEL.
✗Single-mode fiber has a larger core diameter than multimode fiber, allowing longer distances.Wrong answer — click to see why▾
Why this is wrong here
Single-mode fiber has a smaller core, not larger. The smaller core reduces dispersion and allows longer reach.
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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Key term
Small Form-factor Pluggable
A Small Form-factor Pluggable (SFP) is a compact, hot-swappable transceiver module used to connect networking devices to fiber optic or copper cables, converting electrical signals into light signals for data transmission.
Key term
Ethernet
Ethernet is a wired networking technology that connects devices like computers, printers, and servers to each other and to the internet using cables.
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