CCNA Network Infrastructure and Connectivity Practice Question
Network Topology
You are connected to R1. The link between R1's GigabitEthernet0/0 and R2's GigabitEthernet0/0 should operate at 1 Gbps full duplex, but the interface is showing errors and only negotiating at 100 Mbps half duplex. Diagnose and fix the fault, then verify the link is stable at the correct speed and duplex.
⚠ Common exam trap
The exam trap is that candidates may think manually setting the correct speed and duplex is always the best approach, but they must consider the remote device's configuration. Auto-negotiation is the default and preferred method for Gigabit Ethernet; manual settings should be used consistently on both ends.
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 the manual speed and duplex settings on R1's GigabitEthernet0/0 with 'no speed' and 'no duplex' to allow auto-negotiation.
The interface was manually configured with 'duplex half' and 'speed 100', which forced the link to 100 Mbps half duplex, causing CRC errors due to duplex mismatch. The correct fix is to remove these manual settings and allow auto-negotiation, or explicitly set both sides to 'speed 1000' and 'duplex full'. Since the remote side (R2) is set to auto (default), the simplest correction is to use 'no duplex' and 'no speed' on R1 to re-enable auto-negotiation. After the commands are applied, the interface should show 'Full-duplex, 1000Mb/s' and CRC errors should stop incrementing.
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 the manual speed and duplex settings on R1's GigabitEthernet0/0 with 'no speed' and 'no duplex' to allow auto-negotiation.
Why this is correct
This is correct because the interface was manually set to 100 Mbps half duplex, causing a duplex mismatch with R2's auto-negotiating interface. Removing these settings restores auto-negotiation, which will match R2's settings and establish 1 Gbps full duplex, eliminating CRC errors.
- ✗
Change the duplex setting to 'full' and speed to '1000' on R1's GigabitEthernet0/0.
Why it's wrong here
This is incorrect because while setting speed 1000 and duplex full would work if R2 were also manually configured, the question states R2 is set to auto. Manually setting speed and duplex on one side while the other side auto-negotiates can still cause a mismatch if the auto side does not match the manual settings correctly. The safer approach is to remove manual settings.
- ✗
Replace the cable between R1 and R2 with a crossover cable.
Why it's wrong here
Replacing the cable is incorrect because Gigabit Ethernet interfaces support Auto-MDIX, which automatically detects and corrects for straight-through vs. crossover cabling. The symptom of CRC errors and the recovery to 1 Gbps full duplex points to a speed/duplex mismatch caused by manually forcing 100 Mbps half duplex on R1 while R2 auto-negotiates. Since Auto-MDIX compensates for cable type, the physical layer connection is not the root cause, and swapping the cable would not resolve the mismatch. The proper fix is to remove the manual 'speed' and 'duplex' commands so both interfaces negotiate identical parameters.
- ✗
Configure R2's GigabitEthernet0/0 with 'speed 100' and 'duplex half' to match R1's settings.
Why it's wrong here
This is incorrect because matching the wrong settings would stabilize the link at 100 Mbps half duplex, but the requirement is for 1 Gbps full duplex. This would not resolve the speed/duplex mismatch but would instead lock both sides into suboptimal settings.
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 the manual speed and duplex settings on R1's GigabitEthernet0/0 with 'no speed' and 'no duplex' to allow auto-negotiation.Correct answer▾
Why this is correct
This is correct because the interface was manually set to 100 Mbps half duplex, causing a duplex mismatch with R2's auto-negotiating interface. Removing these settings restores auto-negotiation, which will match R2's settings and establish 1 Gbps full duplex, eliminating CRC errors.
✗Change the duplex setting to 'full' and speed to '1000' on R1's GigabitEthernet0/0.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that manually setting speed and duplex on one side while the other side is set to auto can lead to a mismatch; auto-negotiation is required for proper link establishment.
Why candidates choose this
Candidates pick this because they know that 1 Gbps full duplex is the desired setting and think explicitly configuring it will fix the issue, but they overlook the importance of consistent configuration on both ends.
✗Replace the cable between R1 and R2 with a crossover cable.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that Auto-MDIX eliminates the need for crossover cables on modern interfaces; cable type is not the cause of the problem.
Why candidates choose this
Candidates pick this because they recall that duplex mismatches can sometimes be caused by incorrect cable types, but in this scenario the cable is not the issue.
✗Configure R2's GigabitEthernet0/0 with 'speed 100' and 'duplex half' to match R1's settings.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that matching the incorrect settings does not achieve the desired speed and duplex; it only prevents errors at a lower performance level.
Why candidates choose this
Candidates pick this because they think consistency between both ends is the only requirement, ignoring the performance goal of 1 Gbps full duplex.
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
CRC
CRC (Cyclic Redundancy Check) is an error-detecting code used to detect accidental changes to raw data in digital networks.
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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JA
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.