Is the motherboard LAN port (RJ45) a 568a or 568b?

[SomeRandomNerd]

I'm pretty sure that it doesn't matter what Cat the cable is if it uses the same RJ45 connector but is the socket on the motherboard a 568a or 568b? Can both work? Does it depend on the motherboard? How would I find out which one it is?

 

Motherboard (B550M Pro4)

[Needfuldoer]

Both and neither! Those standards apply to the cabling, not the port on the interface.

 

If you have a cable that's the same standard on both ends, it's a straight-through patch. If the cable has A on one end and B on the other, it's a crossover. (The "Transmit" and "Receive" pairs get crossed for 10/100.)

 

Generally we don't have to worry about that anymore, because Auto MDI-X is a thing and modern NICs will figure out the best way to talk to each other as part of the handshake process. You only really need crossover cables if that process fails, or you're working with an old 10/100 interface that doesn't do Auto MDI-X.

 

Generally, in North America everyone uses 568b on everything, just for consistency.

Edited June 24, 2022 by Needfuldoer
Clarification because brwainer yelled at me :(

[mr fobs]

It is supposed to support both (to my knowledge), but I always do my ethernet as 568b.

[Mayday793]

When I used to work in IT here in Vancouver Canada, I learned from my boss that in Canada most people use 568a because Canadians say "eh". 

In the states they use 568b. But back to the question: both work! 

[brwainer]

23 minutes ago, Needfuldoer said:

If the cable has A on one end and B on the other, it's a crossover. (The "Transmit" and "Receive" pairs get crossed.)

Please…. Please…. Stop repeating this statement as-is.

 

If you look at 568a and 568b closely, you can see that a cable made like this is only crossing over two of the four pairs. A true crossover cable swaps all four pairs.

 

The reason this myth happens is because such a cable does actually work as a crossover cable for 10/100 ethernet. But if you use it with gigabit devices, it won’t work.

 

Most gigabit devices have Auto-MDIX, sometimes called Auto-Crossover. They have circuitry and logic to flip the pairs as needed. However, this only works for all four pairs at once. If you use an A/B cable with Auto-MDIX enabled devices, they will keep flipping around, and probably end up defaulting to a link speed of 10/half (the crossover detection is done at the same time and with the same logic as the speed and duplex detection).

 

Early gigabit devices, most notably the uplink ports on Cisco 2950 series switches, do not have Auto-MDIX. An A/B cable used between these will not link at gigabit. If you buy or make a true crossover cable for use with this type of device, it will have all four pairs crossed over - one of the ends will be neither 568a nor 568b.

[Needfuldoer]

3 hours ago, brwainer said:

Please…. Please…. Stop repeating this statement as-is.

Amended.

 

To be fair, there's an extremely remote chance of running into those edge cases in a residential setting. If you've got modern consumer Gigabit gear, you can just use a plain old patch and it will work regardless. If you really need an A/B crossover on a home LAN, it'll be for old 10/100 gear.

[brwainer]

2 minutes ago, Needfuldoer said:

Amended.

 

To be fair, there's an extremely remote chance of running into those edge cases in a residential setting. If you've got modern consumer Gigabit gear, you can just use a plain old patch and it will work regardless. If you really need an A/B crossover on a home LAN, it'll be for old 10/100 gear.

Or…. You can purchase a proper crossover cable for the same price as a regular patch cable, and not leave something crimped in the wall/panel that is not a straight-through, which is going to really mess with someone else or even yourself in the future who can’t figure out why the wire doesn’t work.

[Needfuldoer]

4 minutes ago, brwainer said:

Or…. You can purchase a proper crossover cable for the same price as a regular patch cable, and not leave something crimped in the wall/panel that is not a straight-through, which is going to really mess with someone else or even yourself in the future who can’t figure out why the wire doesn’t work.

Yeah, always stick with one standard for your horizontal runs.