CCNA Network Infrastructure and Connectivity Practice Question
Network Topology
You are connected to R1. The link between R1 and R2 is experiencing packet loss and slow performance. Examine the following partial show interface output:
R1# show interfaces gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Duplex: Full, Speed: Auto, 100Mb/s
Input errors: 12345, CRC: 5000, frame: 0, overrun: 0, ignored: 0
R2# show interfaces gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Duplex: Half, Speed: Auto, 100Mb/s
Input errors: 0, CRC: 0, collisions: 5000, late collisions: 0
Identify the root cause of the issue, and apply the necessary fix on R1 to resolve the problem.
⚠ Common exam trap
A common mistake is to misinterpret the presence of CRC errors on R1 as indicating a cable fault. In a duplex mismatch, the full-duplex side (R1) experiences collisions that manifest as CRC errors (since it does not use CSMA/CD), while the half-duplex side (R2) shows collisions but no CRC errors. Therefore, high CRC errors on one side with no CRC errors but many collisions on the other side is a classic sign of a duplex mismatch, not a physical layer issue.
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 the interface with 'duplex half' to match the half-duplex setting on R2.
The output reveals a duplex mismatch. R1 is operating in full‑duplex mode, while R2 is in half‑duplex mode. On the full‑duplex side (R1), simultaneous transmissions from both ends result in corrupted frames, visible as a high count of CRC errors. On the half‑duplex side (R2), ordinary collisions occur because R2 uses CSMA/CD, but no CRC errors are seen because it detects collisions and retransmits. The only immediate fix from R1—since R2 is fixed at half‑duplex—is to change R1's interface to half‑duplex using the command 'duplex half', which matches the settings and stops the corrupted frames. (Long‑term, both sides should ideally be set to full‑duplex, but that requires access to R2.)
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 the interface with 'duplex half' to match the half-duplex setting on R2.
Why this is correct
This is correct because the symptoms (high input errors without CRC errors on R1) indicate a duplex mismatch, with R1 set to full and R2 likely set to half. Setting R1 to half-duplex matches the duplex setting on R2, eliminating collisions and packet loss.
- ✗
Configure the interface with 'speed 100' to force the link speed to 100 Mbps.
Why it's wrong here
Forcing 'speed 100' only corrects a speed mismatch, but the interface is already running at the same speed; the real issue is that R1 is set to full-duplex while R2 is half-duplex. A duplex mismatch causes hidden collisions and input errors because the half-duplex side cannot detect an idle medium before transmitting, so changing only the speed leaves the underlying fault untouched. You must align the duplex mode, not just the speed.
- ✗
Replace the Ethernet cable with a new one to fix the physical layer issue.
Why it's wrong here
A faulty Ethernet cable typically produces CRC errors, alignment errors, runts, or giants due to electrical signal degradation or defective pins, not the pattern of high input errors with a healthy CRC counter. In this scenario, the error signature—where frames are discarded on the full-duplex side because the half-duplex side aborts them after detecting collisions—is the classic duplex mismatch. Replacing the cable would not alter the fact that R2 remains half-duplex while R1 stays full-duplex, so the errors would persist.
- ✗
Configure the interface with 'no shutdown' to bring the interface up.
Why it's wrong here
The interface output shows the port is administratively up and line protocol is up, so 'no shutdown' is unnecessary and would have no impact on the rising input error counters. A duplex mismatch is a logical configuration issue that disrupts data transfer while the link itself remains physically active; this is not an interface-shutdown problem. The correct resolution is to reconfigure the duplex setting, not to toggle the administrative state.
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 the interface with 'duplex half' to match the half-duplex setting on R2.Correct answer▾
Why this is correct
This is correct because the symptoms (high input errors without CRC errors on R1) indicate a duplex mismatch, with R1 set to full and R2 likely set to half. Setting R1 to half-duplex matches the duplex setting on R2, eliminating collisions and packet loss.
✗Configure the interface with 'speed 100' to force the link speed to 100 Mbps.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that speed mismatch does not cause the high input error count without CRC errors; duplex mismatch does.
Why candidates choose this
Candidates might think that packet loss is due to speed mismatch, especially if they see 'speed' in the output, but the error pattern points to duplex.
✗Replace the Ethernet cable with a new one to fix the physical layer issue.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that cable issues produce CRC errors, not just input errors; the absence of CRC errors rules out cable problems.
Why candidates choose this
Candidates often jump to cable replacement when they see errors, but the specific error type (input errors without CRC) points to duplex mismatch.
✗Configure the interface with 'no shutdown' to bring the interface up.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that 'no shutdown' is used to enable an interface that is administratively down; here the interface is up and passing traffic.
Why candidates choose this
Candidates might think the interface is down due to errors, but the output shows it is up; they may confuse error counters with interface state.
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?”
Visual reference
Go deeper
Related to this question
Learn chapter
ARP — Address Resolution Protocol
Key term
Cyclic Redundancy Check
A Cyclic Redundancy Check (CRC) is an error-detecting code used to verify that data transmitted over a network or stored on a device has not been corrupted.
Key term
Frame
A frame is a unit of data at the Data Link layer of networking, containing header, payload, and trailer information for local network delivery.
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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.