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Difficulty: MediumInterface and Cable Troubleshooting Issues

An engineer is troubleshooting a degraded Ethernet connection on switch interface `FastEthernet0/8` connected to an end host. The interface output reveals high numbers of `late collisions` and `deferred` frames, while `input errors` remain at zero:

text
FastEthernet0/8 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 000c.ce6b.a108
Full-duplex, 100Mb/s, media type is 100BaseTX
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 buffer
3842 late collisions, 1045 deferred, 0 lost carrier

Which TWO physical or data-link conditions are the most probable causes of these output statistics? (Select TWO.)

  1. A duplex mismatch where the switch port is forced to full-duplex and the connected device is operating in half-duplexAnswer
  2. The copper cable segment length exceeds the maximum 100-meter limit defined by Ethernet standardsAnswer
  3. C
    A native VLAN identifier mismatch between the switch port and the connected end-user host
  4. D
    An incorrect cable pinout selection (straight-through instead of crossover) between legacy switch ports without Auto-MDIX

Answer

The most probable causes are a duplex mismatch where one side operates in full-duplex while the other operates in half-duplex, and an Ethernet copper cable run that exceeds the 100-meter maximum length specification.
Late collisions occur when a device detects a collision after transmitting the first 64 bytes of an Ethernet frame. The two primary causes are a duplex mismatch (where one end operates in full-duplex while the other operates in half-duplex, causing the half-duplex end to detect late collisions) and an over-length cable run (where cable propagation delay exceeds the slot time of 512 bit times).

Step-by-Step Solution

1
Analyze the interface statistics provided in the CLI output
Identified high numbers of late collisions (3842) and deferred frames (1045), with zero CRC or input errors.
Late collisions occur when a collision is detected after the first 64 bytes (512 bits) of a frame have been transmitted.
2
Evaluate potential causes of late collisions on Ethernet links
Duplex mismatches (full-duplex on switch, half-duplex on host) cause the half-duplex host to detect full-duplex transmissions as late collisions. Excessively long cable runs (>100m) delay collision detection past the 64-byte window.
Late collisions are exclusively associated with timing issues on half-duplex collision domains or duplex mismatches.
3
Eliminate invalid options based on Layer 1/Layer 2 symptom profiles
Native VLAN mismatches affect traffic routing/tagging across trunks rather than interface collision counters. Cable pinout mismatches without Auto-MDIX cause the link status to remain down/down.
Physical cabling pinout errors prevent link layer synchronization entirely.

Key Concept

Late Collision and Duplex Mismatch Troubleshooting
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Topics

Interface and Cable Troubleshooting Issues

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