CCNA Network Infrastructure and Connectivity Practice Question
Network Topology
You are connected to R1 via console. The link between R1 and R2 is experiencing packet loss and CRC errors. Configure interface speed and duplex on R1's GigabitEthernet0/0 to match R2's settings, then replace the SFP module with one that supports the required 2 km distance. Finally, verify the interface is operational without errors.
⚠ Common exam trap
Do not forget to disable auto-negotiation when manually setting speed and duplex on a Cisco interface. Also, remember that SFP types have specific distance limitations: SX for short reach (550m), LX/LH for long reach (10 km), and EX or ZX for even longer distances.
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
✓
Configure speed 1000, duplex full, and no negotiation auto on Gi0/0, then replace the SFP-GE-SX with SFP-GE-L.
The CRC errors indicate a duplex mismatch or faulty medium. R1 is set to auto-negotiation while R2 is hardcoded to 1000/full, causing mismatch. First, set speed and duplex on R1 to match R2: 'speed 1000' and 'duplex full'. Also disable auto-negotiation with 'no negotiation auto'. The existing SFP-GE-SX only supports 550m, but the link requires 2 km; replace it with a 1000BASE-LX/LH SFP (SFP-GE-L) which supports up to 10 km. After changes, verify with 'show interfaces Gi0/0' to confirm no CRC errors and correct speed/duplex.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure speed 1000, duplex full, and no negotiation auto on Gi0/0, then replace the SFP-GE-SX with SFP-GE-L.
Why this is correct
This is correct because R1 must match R2's hardcoded 1000/full settings. Disabling auto-negotiation with 'no negotiation auto' ensures no mismatch. The SFP-GE-SX only supports 550m, so replacing it with SFP-GE-L (1000BASE-LX/LH) supports the required 2 km distance.
- ✗
Configure speed 1000 and duplex full on Gi0/0, then replace the SFP-GE-SX with SFP-GE-SX-MM.
Why it's wrong here
The speed 1000 and duplex full commands correctly match R2's fixed settings, so the interface configuration side is sound. However, the choice of SFP-GE-SX-MM is still a multimode, short-reach optical module limited to roughly 550 meters on multimode fiber, not the 2 km required. Over a 2 km run, the optical signal will suffer excessive loss due to attenuation and modal dispersion, causing the link to flap or never come up; the correct optic is the single-mode SFP-GE-L (1000BASE-LX/LH).
- ✗
Configure speed 100 and duplex full on Gi0/0, then replace the SFP-GE-SX with SFP-GE-L.
Why it's wrong here
Although swapping the SFP-GE-SX for the SFP-GE-L correctly addresses the 2 km reach, setting speed to 100 and duplex to full creates an immediate speed mismatch with R2's hardcoded 1000 Mbps configuration. Since R2 has auto-negotiation disabled, it will not fall back to 100 Mbps; the link will either remain down or fail to establish. Additionally, the SFP-GE-L is a 1000BASE-LX/LH optic rated for gigabit, and forcing 100 Mbps on it is not supported on Cisco interfaces, leading to configuration errors or an unusable port.
- ✗
Configure no negotiation auto on Gi0/0, then replace the SFP-GE-SX with SFP-GE-L.
Why it's wrong here
Issuing only 'no negotiation auto' without explicitly configuring speed and duplex leaves the interface in its default state, which is typically 1000 Mbps but with duplex still dependent on the SFP type and platform defaults. Because R2 is hardcoded with 1000/full and auto-negotiation disabled, failing to pin R1 to full duplex can result in a duplex mismatch—commonly the local side dropping to half-duplex—which leads to late collisions, FCS errors, and severely degraded throughput. While the SFP-GE-L replacement is correct for the 2 km distance, the missing speed/duplex commands make the link non-deterministic and unreliable.
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.
✓Configure speed 1000, duplex full, and no negotiation auto on Gi0/0, then replace the SFP-GE-SX with SFP-GE-L.Correct answer▾
Why this is correct
This is correct because R1 must match R2's hardcoded 1000/full settings. Disabling auto-negotiation with 'no negotiation auto' ensures no mismatch. The SFP-GE-SX only supports 550m, so replacing it with SFP-GE-L (1000BASE-LX/LH) supports the required 2 km distance.
✗Configure speed 1000 and duplex full on Gi0/0, then replace the SFP-GE-SX with SFP-GE-SX-MM.Wrong answer — click to see why▾
Why this is wrong here
The SFP-GE-SX-MM is a multimode SFP with the same distance limitation as the original SFP-GE-SX.
Why candidates choose this
Candidates might think 'MM' stands for 'more' or 'medium' and assume it supports longer distances, but it actually stands for multimode.
✗Configure speed 100 and duplex full on Gi0/0, then replace the SFP-GE-SX with SFP-GE-L.Wrong answer — click to see why▾
Why this is wrong here
The speed must match exactly; 100 Mbps is not compatible with 1000 Mbps.
Why candidates choose this
Candidates might confuse speed settings or think that 100/full is a common setting for older links.
✗Configure no negotiation auto on Gi0/0, then replace the SFP-GE-SX with SFP-GE-L.Wrong answer — click to see why▾
Why this is wrong here
When auto-negotiation is disabled, speed and duplex must be manually set; otherwise, the interface may default to half-duplex or other incompatible settings.
Why candidates choose this
Candidates might think that disabling auto-negotiation alone is sufficient to match a hardcoded interface, but the speed and duplex must also be explicitly configured.
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?”
Go deeper
Related to this question
Learn chapter
Half-Duplex vs Full-Duplex
Key term
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
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.
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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.