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I'm studying for the CCNA and this PDF talks about knowing straight-though and crossover and where they should be used.

Do Ethernet types T568A and T568B still matter in networking today?

I was always under the impression that devices just auto knew.

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    – Ron Maupin ♦
    Aug 7, 2017 at 21:49

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Crossover vs Straight-through was a bigger deal in the 100BASE-TX days.

Gigabit Ethernet, or 1000BASE-T, requires the use of Auto-MDX, which automatically determine which pairs the opposite side is using for TX/RX and adjusts their side to accommodate.

So if your deployment is largely using Fast Ethernet, then Crossover vs Straight-through is more significant in your environment. If you've moved on to Gigabit ethernet and Fiber, then the distinction is less significant.

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    Even in Fast Ethernet, most interfaces are nowodays auto-mdx, so it become increasingly rare to need a crossover cable, however I always keep one in my bag...
    – JFL
    Sep 12, 2016 at 6:15
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    Don't forget old hardware running some of todays networks. Not every company replaces switches after a couple of weeks.
    – Peter
    Sep 12, 2016 at 8:02
  • @JFL What's interesting is I have two servers here that are Auto-MDX (dual interface, even), but connecting them directly together with a straight-through doesn't work. Curiously, a cross-over works just fine.
    – Der Kommissar
    Dec 7, 2016 at 20:03
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I've come across Cisco router ports not supporting auto-MDX and recently a printer directly connected to a newer laptop with gigabit.

Understanding 568A vs B is important in other ways.

  • A facility will generally standardize on one or the other and punch patch panels and wall jacks to match this standard throughout the facility or campus. All of our locations are 568A. As a medical facility, there is still 100 and 10Mbit equipment. If we varied, we would have mixed results.

  • Understanding the pairs and what they do will help you troubleshooting Layer 1 issues.

  • In some standards, all 4 pairs are used.

  • When learning you might come across older equipment to lab things up and practice. This is where you may come across Fast Ethernet equipment that is detailed by Peter and Eddie above.

  • Patch Cables don't matter - They are not 568A or 568B. Sure the colors may lie that way, but they are just a patch cable and as long as the pairs line up to Ethernet standard, you are fine.

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It depends what kit you are using. You need to worry about straight through VS crossover if neither end supports auto-mdix.

In my experiance:

  • All gigabit gear i've seen supports auto-mdix.
  • Most 100 megabit NICs do not support auto-mdix, an exception is the LAN951x series (which is used in a number of well-known SBCs).
  • Every 100 megabit unmanaged switch I've seen supports auto-mdix.
  • Ports on managed networks may not support auto-mdix. Whether this is because the equipment doesn't support it or the admins disabled it (maybe trying to make it harder for users to accidently create loops) I do not know.
  • I've never seen 10 megabit gear that supports auto-mdix.
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