9

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
  • 153,128
  • 77
  • 353
  • 394
Mike
  • 93
  • 3
  • 2
    According to https://www.cables-solutions.com/difference-between-straight-through-and-crossover-cable.html having A on one end and B on the other is a crossover. Having the same on both ends is straight. https://www.guru99.com/difference-between-straight-through-crossover-cables.html suggests the same just looking at the pinout, https://www.comparitech.com/net-admin/difference-between-straight-through-crossover-rollover-cables/ 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 '23 at 10:01
  • 3
    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 '23 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. – u1686_grawity Jan 26 '23 at 20:42

2 Answers2

10

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
  • 715
  • 4
  • 11
  • 4
    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 '23 at 10:09
0

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
  • 1
  • Third-party exams and other materials are not authoritative on how an Ethernet cable is wired; the IEEE 802.3 [specification](https://i.stack.imgur.com/IA2rS.png) is. Exams are only written to match the standards, not the other way around. – u1686_grawity Apr 03 '23 at 14:46
  • 1
    _"assuming ethernet standards, that are clearly not specified in the problem"_ - the question starts with _"In modern Ethernet networks…"_. It's not precise and should be written better, but they're technically correct. – gronostaj May 30 '23 at 05:45