CCNA Network Infrastructure and Connectivity Practice Question
Network Topology
You are troubleshooting a link between R1 (G0/0) and R2 (G0/0). The link is up but experiencing packet loss. You suspect an interface speed/duplex mismatch or SFP issue. Configure R1's interface to match the correct speed and duplex, and replace the SFP module if necessary to support a required distance of 40 km over single-mode fiber.
⚠ Common exam trap
This question tests your ability to identify and resolve both a speed/duplex mismatch and an SFP distance limitation. A common trap is to focus on only one issue (e.g., only fixing the mismatch or only replacing the SFP) while ignoring the other. Always verify that the SFP supports the required distance and fiber type.
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
✓
Remove manual speed/duplex settings, enable auto-negotiation, and replace the SFP with a long-reach module (e.g., 1000BASE-ZX) that supports 40 km over single-mode fiber.
The link is experiencing packet loss, likely due to a speed/duplex mismatch between the manually configured interface and the peer (which may use auto-negotiation) and/or an SFP module that cannot support the required 40 km distance over single-mode fiber. To resolve both potential issues, remove any manual speed and duplex settings to restore default auto-negotiation (using 'no speed' and 'no duplex'), and replace the SFP with a module rated for 40 km or more on SMF, such as a 1000BASE-ZX (or 1000BASE-EX) transceiver. Standard 1000BASE-LX/LH only reaches up to 10 km on SMF, so it is insufficient for this scenario.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Remove manual speed/duplex settings, enable auto-negotiation, and replace the SFP with a long-reach module (e.g., 1000BASE-ZX) that supports 40 km over single-mode fiber.
Why this is correct
This is the correct solution because it resolves both independent issues. Removing the manual speed/duplex configuration and restoring auto-negotiation ensures the two Cisco interfaces agree on speed and duplex, eliminating any negotiation mismatch. Replacing the 10 km SFP with a long-reach 1000BASE-ZX module (1550 nm) increases the optical power budget to support distances of 70-80 km on single-mode fiber, comfortably satisfying the 40 km requirement. The combination of logical parameter agreement and proper physical optics is necessary to bring the link up reliably over the intended span.
- ✗
Change the duplex to full and speed to 1000 Mbps manually, and keep the existing SFP.
Why it's wrong here
This is incorrect because manually setting speed and duplex disables auto-negotiation, which can still cause mismatch if the remote end is set differently. Moreover, the existing SFP is rated for only 10 km, not the required 40 km, so it must be replaced.
- ✗
Keep the manual speed/duplex settings but replace the SFP with a 1000BASE-SX module.
Why it's wrong here
This is incorrect because keeping manual settings does not resolve the mismatch; the link partner likely uses auto-negotiation or different settings. Also, 1000BASE-SX is designed for multimode fiber (up to 550 m), not single-mode fiber, and cannot support 40 km.
- ✗
Enable auto-negotiation on both ends but keep the existing SFP.
Why it's wrong here
Enabling auto-negotiation on both ends will resolve the speed and duplex mismatch, as the link partners will exchange capabilities and converge on common parameters. However, the existing SFP has a reach rating of only 10 km on single-mode fiber, while the specified 40 km link far exceeds its optical power budget. Even with auto-negotiation, the received signal will fall below the receiver sensitivity threshold, causing high bit error rates or the link simply never coming up. Therefore, the SFP must be replaced to meet the distance requirement.
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.
✓Remove manual speed/duplex settings, enable auto-negotiation, and replace the SFP with a long-reach module (e.g., 1000BASE-ZX) that supports 40 km over single-mode fiber.Correct answer▾
Why this is correct
This is the correct solution because it resolves both independent issues. Removing the manual speed/duplex configuration and restoring auto-negotiation ensures the two Cisco interfaces agree on speed and duplex, eliminating any negotiation mismatch. Replacing the 10 km SFP with a long-reach 1000BASE-ZX module (1550 nm) increases the optical power budget to support distances of 70-80 km on single-mode fiber, comfortably satisfying the 40 km requirement. The combination of logical parameter agreement and proper physical optics is necessary to bring the link up reliably over the intended span.
✗Change the duplex to full and speed to 1000 Mbps manually, and keep the existing SFP.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that manually setting speed/duplex does not guarantee compatibility with the remote end and does not address the SFP distance limitation.
Why candidates choose this
Candidates might think that manually forcing 1000 Mbps full-duplex will fix the mismatch, but they overlook the SFP requirement and the risk of mismatch if the remote end uses auto-negotiation.
✗Keep the manual speed/duplex settings but replace the SFP with a 1000BASE-SX module.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that 1000BASE-SX is for multimode fiber and short distances, not single-mode fiber at 40 km.
Why candidates choose this
Candidates might confuse SFP types and think any Gigabit SFP will work, but they ignore the fiber type and distance limitations.
✗Enable auto-negotiation on both ends but keep the existing SFP.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the SFP distance limitation is not addressed; auto-negotiation alone does not change the physical layer capability.
Why candidates choose this
Candidates might focus only on the mismatch issue and forget the distance requirement, assuming the SFP is adequate.
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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Related to this question
Learn chapter
Half-Duplex vs Full-Duplex
Key term
Packet loss
Packet loss is the failure of one or more data packets to reach their destination across a network, resulting in missing or incomplete data transfers.
Key term
Duplex mismatch
A duplex mismatch occurs when two connected network devices are configured with different duplex settings (one half-duplex, one full-duplex), leading to poor performance and data errors.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.