Causes of Late Collisions on Ethernet Interfaces

200-301 By islever Editorial Last updated

Identify interface and cable issues (collisions, errors, mismatch duplex, and/or speed) Explain the role and function of network components

Short answer

BE Late collisions are caused by duplex mismatches and exceeded cable length limits.

Question

What are two reasons that cause late collisions to increment on an Ethernet interface? (Choose two.)
  1. when Carrier Sense Multiple Access/Collision Detection is used
  2. when one side of the connection is configured for half-duplex Correct Answer
  3. when the sending device waits 15 seconds before sending the frame again
  4. when a collision occurs after the 32nd byte of a frame has been transmitted
  5. when the cable length limits are exceeded Correct Answer

Key Takeaways

Late collisions occur after the first 64 bytes; they are distinct from normal collisions and are typically caused by physical layer issues like cable length or configuration errors like duplex mismatch.

Identify the two main reasons for late collisions: duplex mismatch and cable length violations. Learn why these factors push collisions beyond the 64-byte threshold.

Common mistake: Many candidates select Option B incorrectly thinking half-duplex itself is the problem, but it is specifically the *mismatch* with a full-duplex neighbor that causes the late collision. Others might choose D, confusing normal collision windows with late ones.

Expert Analysis

Late collisions are a specific type of Ethernet error defined by the timing of the collision relative to the frame transmission. In IEEE 802.3, the slot time is 512 bits (64 bytes). A collision that occurs within this first 64 bytes is considered normal and is handled by the CSMA/CD protocol's backoff algorithm. However, if a collision happens after the first 64 bytes have been transmitted, it is classified as a 'late collision.' This indicates that the network topology or physical layer constraints prevent the collision signal from reaching the sender in time for standard handling.

The two primary causes for late collisions are duplex mismatches and excessive cable lengths. When one side of a link is configured for half-duplex and the other for full-duplex (Option B), the full-duplex side transmits without listening for collisions, while the half-duplex side listens and may transmit simultaneously. Because the full-duplex device ignores the carrier sense, it does not back off, leading to a collision that often occurs well into the frame transmission, resulting in a late collision counter increment. Similarly, if the cable length exceeds the specified limits for the media type (Option E), the propagation delay increases. If the distance is too great, the signal takes longer to travel between nodes than the slot time allows, meaning a collision detected at the far end will be reported back to the source only after the 64-byte threshold has passed.

Options A and D are incorrect because CSMA/CD is the mechanism designed to manage collisions, not cause them, and collisions within the first 32 bytes are normal, not late. Option C refers to retransmission timers, which are unrelated to the physical detection of late collisions.

Exam Strategy

When analyzing interface counters, always correlate the counter name with the OSI layer. 'Collision' implies Layer 1/2. Differentiate between 'normal' collisions (handled by CSMA/CD) and 'late' collisions (physical defects or mismatches). Remember that late collisions (>64 bytes) are almost always due to propagation delays exceeding the slot time, caused by either distance (cable length) or asymmetric behavior (duplex mismatch).

Frequently Asked Questions

What is the byte threshold for a late collision?

A late collision is any collision that occurs after the first 64 bytes (512 bits) of the frame have been transmitted.

Can full-duplex links experience collisions?

No, full-duplex links disable CSMA/CD, allowing simultaneous transmit and receive without collision detection. Collisions only occur on half-duplex links or during duplex mismatches.

More 200-301 FAQ →

Community Votes

E
50%
B
43%
A
7%

50% of anonymous learners picked answer E. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 37 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 30 of 37

  1. John248 👍 29
    Directly from a Cisco article. What are two reasons that cause late collisions to increment on an Ethernet interface? (Choose two) A. when the sending device waits 15 seconds before sending the frame again B. when the cable length limits are exceeded C. when one side of the connection is configured for half-duplex D. when Carrier Sense Multiple Access/Collision Detection is used E. when a collision occurs after the 32nd byte of a frame has been transmitted Answer: B, C A late collision is defined as any collision that occurs after the first 512 bits (or 64th byte) of the frame have Been transmitted. The usual possible causes are full-duplex/half-duplex mismatch, exceeded Ethernet cable length limits, or defective hardware such as incorrect cabling, non-compliant number of hubs in the network, or a bad NIC. Late collisions should never occur in a properly designed Ethernet network. They usually occur when Ethernet cables are too long or when there are too many repeaters in the network. Reference: https://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html
  2. ZUMY 👍 10
    Given Answer B & E are correct! * A late collision is defined as any collision that occurs after the first 512 bits (or 64th byte) of the frame have Been transmitted. The usual possible causes are full-duplex/half-duplex mismatch, exceeded Ethernet cable length limits, or defective hardware such as incorrect cabling, non-compliant number of hubs in the network, or a bad NIC. Late collisions should never occur in a properly designed Ethernet network. They usually occur when Ethernet cables are too long or when there are too many repeaters in the network.
  3. dendio 👍 6
    This is correct, but the answers have been moved around
  4. vkv1975 👍 5
    B & E ARE CORRECT ANSWERS
  5. shurik04 👍 3
    A late collision is defined as any collision that occurs after the first 512 bits (or 64th byte) of the frame have been transmitted. The usual possible causes are full-duplex/half-duplex mismatch, exceeded Ethernet cable length limits, or defective hardware such as incorrect cabling, non-compliant number of hubs in the network, or a bad NIC. Late collisions should never occur in a properly designed Ethernet network. They usually occur when Ethernet cables are too long or when there are too many repeaters in the network.
Show 25 more comments
  1. Shaz313 👍 3
    Late Collision is a collision on an Ethernet network that is detected late in the transmission of the packet. Late collisions can result from defective Ethernet transceivers, from having too many repeaters between stations, or from exceeding Ethernet specifications for maximum node-to-node distances the right answer is B_E
  2. iRodimusPrime 👍 3
    I would go with A and E here... the counters wouldn't increase on both devices, just the one in half-duplex. That's if the link went up at all...
  3. Hodicek 👍 3
    B- E is the correct answer, search on google on the 2 reasons that cause late collision B- E is the correct answer 100%
  4. Kaygol 👍 3
    I say B and E, but please can staff correct it then, so it shows the right answer?
  5. Lakshmi_200_301 👍 3
    I think questions B and E are correct answers
  6. SALSHOUMER 👍 3
    the right answer is B_E
  7. admin1982 👍 3
    Definitely B and E
  8. rlelliott 👍 2
    The late collision counter is increased whenever a late collision is detected on the link, this means the collision happened after the 64th byte has been transmitted. A duplex mismatch on the link will always allow the links to come up, however it will usually generate duplex mismatch error messages on both devices. It also can and will cause late collisions to occur on the link. A cable exceeding 328 ft an also cause late collisions although not always and depending on the overall length of the cable run. B and E are the correct answers.
  9. jowill 👍 2
    B is not a correct answer because at CSMA/CD mode, end points cannot send and receive frames at the same time. Therefore end points(NIC) have to be in half-duplex mode. B is not a cause of late collision. But A can detect collision but also not the cause of late collision. All in all there is issue in the description of the question itself.
  10. Nmk173 👍 2
    Correct is B and E. B . If you use Half -Dublex it cause collusion. It doesnt matter both side is half-dublex. If it is full dublex than no collusion. E. This is clear. If cable is long than limit, it cause late collision. Why not A? Because CSMA/CD is not cause. It is for to stop collusion. It is the reason..
  11. khalid86 👍 2
    I have seen David Bombal videos. He demonstrated on GNS3 the late collision is also the result of duplex mismatch. And I have also seen from other videos that CSMA/CD was being used in older networks in bus topology. Answer is B,E
  12. boghota 👍 2
    But B (when one side of the connection is configured for half-duplex) leaves open if the other side of the connection is configured as half-duplex as well so this doesn´t necessarily mean a duplex missmacth or am I wrong here?
  13. whojabagooya 👍 2
    I'm going to have to agree with the cited cisco literature. They specifically site long cables and repeaters which are half duplex.
  14. stevel 👍 2
    Is it A and E or B and E, guys. I think it's B and E, too, btw. But please confirm and if wrong then correct the answer here.
  15. therandman 👍 2
    I understand what a collision is but what is a "late" collision. I've never heard this term before. Is it a Cisco term?
  16. Whippy29 👍 2
    cisco courseware Therefore, collisions, especially late collisions, often indicate duplex mismatches
  17. sarsat 👍 2
    B& E https://learningnetwork.cisco.com/s/question/0D53i00000Kt6ge/half-or-full-duplex-collisions
  18. daslux4 👍 2
    B and E certainly
  19. devildog 👍 2
    It's B and E
  20. siva_13 👍 2
    B and E
  21. Grande 👍 1
    Duplex mismatch: -- this portion answers your question @therandman The end result is a connection that is working but performs extremely poorly because of the duplex mismatch. Symptoms of a duplex mismatch are connections that seem to work fine with a ping command, but "lock up" easily with very low throughput on data transfers; the effective data transfer rate is likely to be asymmetrical, performing much worse in the half-duplex to full-duplex direction than the other. In normal half-duplex operations late collisions do not occur. However, in a duplex mismatch the collisions seen on the half-duplex side of the link are often late collisions. The full-duplex side usually will register frame check sequence errors, or runt frames.
  22. boghota 👍 1
    But on the other hand, as Wikipedia states: "As a correctly set up CSMA/CD (Carrier-sense multiple access with collision detection) network link should not have late collisions, the usual possible causes are full-duplex/half-duplex mismatch, exceeded Ethernet cable length limits, or defective hardware such as incorrect cabling, non-compliant number of hubs in the network, or a bad NIC." So I guess A can not be the right answer. https://en.wikipedia.org/wiki/Carrier-sense_multiple_access_with_collision_detection#Late_collision
  23. lohaN73 👍 1
    CSMA/CD happens before any collision,not after. So,option A can be kicked out at first glance... option B & E are valid
  24. harunrasit 👍 1
    The answer is BE. Half -Dublex it cause collusion. If cable is long than limit, it cause late collision.
  25. illuded03jolted 👍 1
    B and E are correct options.

Reader comments & corrections

No reader corrections yet — spotted an error or have a note? Share it below.

Log in to comment, report an error, or add a note about this question.

Rapid PVST+ Port State That Neither Receives BPDUs nor Learns MAC

S-grade

Computing the EUI-64 Interface ID for a Given MAC Address

S-grade

Electing the Root Bridge in PVST+ by Priority and MAC

S-grade

How a Switch Handles an ARP Request for an Unknown Destination

S-grade

Practice All 200-301 Questions

Access 220 questions with complete answers and detailed explanations.

View Full 200-301 Practice Test →

← Back to 200-301 Study Guide