Domain 5 of 5 · Chapter 2 of 5

Cabling and Physical Issues

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Included in this chapter:

  • Cable faults: type, distance, EMI, and termination
  • Interface counters: CRC, runts, giants, and late collisions
  • Hardware faults: PoE budget, transceivers, and fiber loss
  • Exam-pattern recognition: symptom, cause, tool

Physical-layer symptom to most likely cause to confirming tool

SymptomMost likely causeConfirming toolFix
Link drops only on long runsExceeds 100 m / attenuationTDR or cable certifier (length)Use fiber or add a switch
Intermittent errors near equipmentEMI / crosstalk (NEXT)Cable certifier (NEXT test)Reroute, shielded cable, or fiber
Reversed/open/short pairImproper termination, wrong pinoutWiremap cable testerRe-terminate to one T568 standard
Dead like-device link, old gearTX/RX transposed, no Auto-MDIXWiremap tester / pinout checkCrossover cable or Auto-MDIX
CRC errors and runts climbingBad cable / transceiver / EMIInterface countersReplace cable or transceiver
Late collisionsDuplex mismatchInterface counters (per side)Match duplex on both ends
PoE device won't power onPoE budget or standard mismatchSwitch PoE power inventoryFree budget or match af/at/bt
Fiber down or high lossTransceiver/wavelength mismatch, dB lossLight meter / OTDRMatch optics, clean/shorten run

Decision tree

Link dead, or slow/lossy?read the symptom firstDead: check the cablecopper or fiber?Slow/lossy: counterswhich counter climbs?no linklink upWiremap testeropen/short/pinoutCrossover / Auto-MDIXTX/RX transposedlike-device, old gearMatch transceiverfiber mode + wavelengthcopper pinoutlike-devicefiber downReplace cable/opticCRC + runts = physicalMatch duplexlate collisionsCRC/runtslate coll.Won't power on? Check PoE budget + standardaf 15.4 W / at 30 W / bt ~90 W
Cheat sheet

Cheat sheet

  • Diagnose physical faults symptom to cause to tool
  • Twisted-pair copper stops at 100 meters regardless of category
  • Attenuation is signal loss over distance
  • EMI corrupts copper frames; fiber is immune
  • Crosstalk and NEXT signal a termination problem
  • Keep one T568 standard on both ends of a straight-through run
  • TX/RX transposed kills a like-device link without Auto-MDIX
  • Auto-MDIX removes the need to choose straight vs crossover
  • CRC errors and runts mean a physical-layer fault
  • Giants point at an MTU or tagging misconfiguration
  • Late collisions are the signature of a duplex mismatch
  • A duplex mismatch is one side full, the other half
  • Compare error counts to total traffic before acting
  • PoE fails when the switch power budget is exhausted
  • Match the PoE standard to the device's power draw
  • A transceiver mismatch keeps a fiber link down
  • Single-mode and multimode optics and fiber must match
  • Fiber health is measured as dB loss against a link budget
  • Port status names the next step: down vs err-disabled
  • Passing wiremap but failing insertion loss points to damage or wrong category
  • A split pair passes wiremap but causes crosstalk and CRC errors
  • A reversed pair swaps pin 1 and pin 2 within one pair

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References

  1. Troubleshoot Switch Port and Interface Problems (CRC, runts, giants, late collisions, duplex, Auto-MDIX)
  2. IEEE 802.3af-2003 — DTE Power via MDI (Power over Ethernet, up to 15.4 W at the source) Whitepaper
  3. IEEE 802.3at-2009 — DTE Power Enhancements (PoE+, up to 30 W at the source) Whitepaper
  4. IEEE 802.3bt-2018 — DTE Power via MDI over 4-Pair (PoE++/4PPoE, up to 90 W at the source) Whitepaper