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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?

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    According to cables-solutions.com/… having A on one end and B on the other is a crossover. Having the same on both ends is straight. guru99.com/… suggests the same just looking at the pinout, comparitech.com/net-admin/… also agrees that straight is either A-A or B-B and crossover is A-B. I'm not sure why you are wrong.
    – Mokubai ♦
    Jan 26 at 10:01
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    It might be that it is "only" a crossover for 100baseTX, which uses 2 pairs, and not a true crossover for gigabit ethernet which uses all four pairs. Auto MDI-X would negotiate pairs for each direction and ignore any crossovers.
    – Mokubai ♦
    Jan 26 at 10:03
  • The 1000BASE-T specification does define a "crossover" wiring... which crosses all four pairs, unlike the old crossover which only did two pairs.
    – user1686
    Jan 26 at 20:42

1 Answer 1

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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

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    This is the only real reason I can see for being "wrong" as well. Everything I can find says that the question is right to call it a crossover, and for 100 Base TX ethernet it is. Since Gigabit uses all four pairs having only two of them swapped is not quite right any more and Auto MDI-X is the way that hardware fixes whatever cabling you might have.
    – Mokubai ♦
    Jan 26 at 10:09

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