CCNA Network Infrastructure and Connectivity Practice Question
Network Topology
You are connected to R1 via the console. R1 and R2 are directly connected via their GigabitEthernet0/0 interfaces. The link between them is down. Your task is to diagnose and fix the issue: R1's interface is configured for 100 Mbps full-duplex, but R2 is using auto-negotiation. Additionally, the link requires a Gigabit Ethernet connection over a distance of 5 km. Configure R1's interface to match R2's settings (auto-negotiation) and then select and install the correct SFP module to support the 5 km distance requirement.
⚠ Common exam trap
Students often forget that auto-negotiation must be enabled on both sides for Gigabit Ethernet; hard-coding one side breaks the link. Also, they may confuse SFP types: SX for short range, LX for long range. Always verify distance requirements when selecting fiber optics.
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 R1 with 'no speed', 'no duplex', and 'negotiation auto' on GigabitEthernet0/0, then replace the SFP module with a 1000BASE-LX SFP.
The link is down because R1 is forcing speed 100 and full-duplex while R2 is using auto-negotiation. When one side is hard-coded and the other is set to auto, auto-negotiation fails and the link does not come up. The fix is to enable auto-negotiation on R1 by removing the manual speed and duplex settings with the 'no speed' and 'no duplex' commands, and then using 'negotiation auto'. For the 5 km distance, a standard 1000BASE-SX SFP (550 m) is insufficient; a 1000BASE-LX SFP (up to 10 km) is required. The candidate must also replace the SFP module with a compatible LX SFP.
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 R1 with 'no speed', 'no duplex', and 'negotiation auto' on GigabitEthernet0/0, then replace the SFP module with a 1000BASE-LX SFP.
Why this is correct
This is correct because removing the hard-coded speed and duplex settings and enabling auto-negotiation on R1 allows the link to come up with R2. For the 5 km distance, a 1000BASE-LX SFP supports up to 10 km, meeting the requirement.
- ✗
Configure R1 with 'speed 1000' and 'duplex full' on GigabitEthernet0/0, then replace the SFP module with a 1000BASE-SX SFP.
Why it's wrong here
This is incorrect because setting speed 1000 and duplex full on R1 while R2 is using auto-negotiation will still cause a mismatch; auto-negotiation fails when one side is hard-coded. Also, 1000BASE-SX only supports up to 550 m, insufficient for 5 km.
- ✗
Configure R1 with 'no speed', 'no duplex', and 'negotiation auto' on GigabitEthernet0/0, then replace the SFP module with a 1000BASE-SX SFP.
Why it's wrong here
Although removing the hard-coded speed and duplex settings and enabling 'negotiation auto' on the GigabitEthernet0/0 interface would correct the link negotiation problem with R2, the choice of 1000BASE-SX SFP is still fatal. The 1000BASE-SX standard operates on multimode fiber and has a maximum segment length of 550 meters, which is far short of the 5 km (5000 m) distance between the routers. Even with perfect auto-negotiation, the optical signal would be too attenuated after 5 km, so the link would never come up. You must use a 1000BASE-LX SFP, which supports single-mode fiber and distances up to 10 km, to meet the physical layer requirement.
- ✗
Configure R1 with 'speed 100' and 'duplex full' on GigabitEthernet0/0, then replace the SFP module with a 1000BASE-LX SFP.
Why it's wrong here
This is incorrect because setting speed 100 on a GigabitEthernet interface forces the link to 100 Mbps, which is not Gigabit Ethernet and will not work with the SFP module (which expects 1000 Mbps). Also, the hard-coded settings will cause a mismatch with R2's auto-negotiation.
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 R1 with 'no speed', 'no duplex', and 'negotiation auto' on GigabitEthernet0/0, then replace the SFP module with a 1000BASE-LX SFP.Correct answer▾
Why this is correct
This is correct because removing the hard-coded speed and duplex settings and enabling auto-negotiation on R1 allows the link to come up with R2. For the 5 km distance, a 1000BASE-LX SFP supports up to 10 km, meeting the requirement.
✗Configure R1 with 'speed 1000' and 'duplex full' on GigabitEthernet0/0, then replace the SFP module with a 1000BASE-SX SFP.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that hard-coding speed and duplex on one side while the other uses auto-negotiation prevents the link from coming up, and 1000BASE-SX cannot reach 5 km.
Why candidates choose this
Candidates might think that setting the same speed (1000 Mbps) will work, but they overlook the duplex mismatch issue and the distance limitation of SX.
✗Configure R1 with 'no speed', 'no duplex', and 'negotiation auto' on GigabitEthernet0/0, then replace the SFP module with a 1000BASE-SX SFP.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that 1000BASE-SX is designed for short-range multimode fiber, not long distances.
Why candidates choose this
Candidates may correctly identify the auto-negotiation issue but fail to consider the distance requirement, assuming any Gigabit SFP will work.
✗Configure R1 with 'speed 100' and 'duplex full' on GigabitEthernet0/0, then replace the SFP module with a 1000BASE-LX SFP.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that GigabitEthernet interfaces can operate at 100 Mbps, but the SFP module requires 1000 Mbps; additionally, the speed/duplex mismatch prevents the link from coming up.
Why candidates choose this
Candidates might think that matching the original speed (100 Mbps) is necessary, but the question states the link requires Gigabit Ethernet, and the SFP module must be used at its rated speed.
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
Ethernet Standards
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
Auto-negotiation
Auto-negotiation is an Ethernet feature that allows two connected devices to automatically agree on the best possible speed and duplex mode for their link.
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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.