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I was going through some A+ practice exams when I came across this question:

In modern Ethernet networks, A twisted-pair copper cable terminated according to the TIA/EIA-568A standard on one end and TIA/EIA-568B standard on the opposite end forms a:

  • Crossover cable
  • Straight-through cable
  • Patch cable
  • None of the above

I answered Crossover cable and was baffled to find that it was incorrect and the correct answer was none of the above.

The only reasoning I can find is that "modern" network devices support Auto MDI-X which would mean the cable used does not matter. However, the phrasing of the question still makes me think crossover cable. Am I missing something here or is this answer invalid?

DavidPostill
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Mike
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2 Answers2

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You will find the statement that mixing the wiring standards would create a crossover cable on various web sites.

Mixing the wiring standards will cross over pairs 1-2 and 3-6 and leave 4-5 and 7-8 straight through. This wrong connection is something like a half-crossover cable.

With older Ethernet standards 100BASE-TX or 10BASE-T, which use only two of the pairs, this would work as a crossover cable.
For 1000BASE-T, which uses all pairs, this is simply wrong wiring.

A correct crossover cable has both pairs 1-2 and 3-6 and pairs 4-5 and 7-8 crossed. This will work with old and new Ethernet standards.

See also https://www.flukenetworks.com/knowledge-base/dsx-cableanalyzer-series/crossover-cable-testing-dsx-cable-analyzer

Bodo
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Short answer: Crossover is A to B. (Non-negotiable) modern or not!

A LOGICAL FALLACY (@Bodo) would be to introduce variables, assuming ethernet standards, that are clearly not specified in the problem.

Per Pearson Vue in accordance with CompTIA N10-008 Network+ Exams; the Crossover cable is defined and depicted as crossing wires 1&3 2&6. There is no literature or image alluding to the cross of blue and brown pairs located at 4&5 and 7&8 respectively.

This is not a troubleshooting question, this is an identification question that should be based directly off the teachings of the Exam.

In troubleshooting this would be readily apparent and more information would be available to the network technicians/administrators.

zach
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