Interface Instability Caused by a Half-Duplex / Duplex Mismatch

200-301 By islever Editorial Last updated

Identify interface and cable issues (collisions, errors, mismatch duplex, and/or speed)

Short answer

B The high 'collisions' counter points to a duplex mismatch (router is full-duplex, ISP side is half-duplex), discarding frames and causing unstable connectivity.

Question

Refer to the exhibit. Router-WAN1 has a new connection via Gi0/0 to the ISP. Users running the web applications indicate that connectivity is unstable to the internet. What is causing the interface issue? - Exhibit: Interface Instability Caused by a Half-Duplex / Duplex Mismatch
  1. The receive buffer is full due to a broadcast storm.
  2. Frames are discarded due to a half-duplex negotiation. Correct Answer
  3. Broadcast packets are rejected because ARP timeout is enabled.
  4. Small frames less than 64 bytes are rejected due to size.

Key Takeaways

Collisions = half-duplex behavior. On a full-duplex link you should see ~0 collisions; a large collision count means the peer is half-duplex — a duplex mismatch.

A rising 'collisions' counter on a link is a classic duplex-mismatch symptom: one side runs full-duplex while the peer is forced to half-duplex. The resulting collisions and CRC/input errors drop frames and make connectivity unstable, exactly the symptom reported.

Common mistake: Blaming a broadcast storm (option A): a storm would inflate broadcast packet counts, but the decisive symptom here is the collisions counter, which is a physical-layer/duplex issue, not a storm.

Expert Analysis

Why the Answer Is Correct

B is correct. The interface shows 100 collisions alongside input errors, which is the signature of a duplex mismatch — the router is full-duplex while the ISP-side port is configured half-duplex. Half-duplex ends experience collisions, drop frames, and produce the unstable connectivity users report.

Why the Other Options Are Wrong

A is wrong: a broadcast storm raises broadcast counts, not the collisions counter. C is wrong: ARP timeout does not reject broadcasts. D is wrong: runt/rejected small frames are not what the collision count indicates.

Community Comment Notes

The vote is B (67) vs A (33). The top comment flags the collisions counter as the key duplex-mismatch indicator; a minority argues broadcast storm, but collisions are the decisive evidence.

Official Reference

https://learningnetwork.cisco.com/s/ccna-exam-topics

This link opens the official vendor documentation in a new tab.

Community Votes

B
67%
A
33%

67% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 4 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 4

  1. sy0_061 👍 6 Answered B
    The collisions are worth looking at. It implies a duplex mismatch. Although the router is full-duplex, the other side might be manually configured for a half-duplex. This causes high number of collisions. Just FYI, broadcast storm is when when we see broadcast packet numbers above 3000. There is a 30% formula but I can't recall it.
  2. Shaifalimittal02 👍 3 Answered A
    The correct answer is A. The receive buffer is full due to a broadcast storm. Here's why: In the output of the show interface command, we see 100 input errors and 260 input packets, which indicates that there may be an issue related to receiving data on the interface. Additionally, there are 110 broadcasts and 100 collisions, which suggests that there is likely a broadcast storm or excessive broadcast traffic, overwhelming the receive buffer. Explanation of incorrect options: B (Half-duplex): The interface is operating in Full Duplex, so half-duplex negotiation is not the issue. C (ARP timeout): ARP type and timeout configuration would not reject broadcast packets. D (Small frames): There is no indication that small frames are being rejected based on size.
  3. bezkin 👍 3
    Answer is B, but don't take my word for it, go learn something Collisions are a strong indicator of half-duplex environment or duplex mismatch Collisions will cause CRC errors and data corruption - another symptom of physical problem (such as duplex mismatch) Frame errors - also a physical layer (duplex mismatch) symptom Input errors - ALSO a physical issue indicator. correlates with duplex mismatch
  4. exiledwl 👍 2
    Correct

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