10/100/1000 ethernet over 2 pair wiring

reabnm used Ask the Experts™
I am troubleshooting a server to clients connectivity issue. I have changed all patch cords and swapped the rj45 jacks. I have not tried installing a new NIC in the server as of yet. However, I did discover today that the cat5 cables at the 12 port patch panel only have the orange and green pairs punched down as per 100 base tx. I wonder if anyone has run into any intermittent connectivity issues when running Gig E on only 2 pair 100 base tx.
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GigE normally run on Cat5 using 4 pair.
Usually Cat5 using 2 pair can only go as fast as 100 BaseT.


Thanks shalvors. I know that GigE requires all 4 pairs, and that 10/100/1000 hardware should throttle down to 100 BaseT if only 2 pairs are present. I was wondering if anyone has experienced or heard of dropped connection issues when running GigE hardware on 2 pair 100 BaseT.
Sr. Network / Systems Admin
In a nutshell, all you need are pins 1,2,3,6 to be on the same post on each end.
That takes care of continuity.

But, to be electrically impeded GigE correctly, Orange/white, Orange, Green/white, Green are the color combos, respectively to 1,2,3,6.

GigE does require all four pairs for proper magnetic impedance.
On your adapter, though, if you enable "Ethernet@Wirespeed" it will force copper to connect at 1Gbps as long as you have at least 1,2,3,6 pinned correctly. If it's not enabled, you will either get no connection, or a 100Mbps connection, depending on brand and / or if you also have speed & duplex negotiation set to "Auto"
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TimotiStDatacenter Technician
Top Expert 2012
After a timeout in autodetection mode, it should fall back to 100Mb mode. But if for any reason (driver bug, temporary disconnect, EMP spike) there's a renegotiation, you can expect a timeout again, which will look like a drop.
Forcing the connection to 100Mb might do the trick, but preferably fix the patch panel issue so it doesn't have to fall back to failure mode, and 1Gb would be nicer for a server, if the clients are 100Mb anyway. :)
Lee W, MVPTechnology and Business Process Advisor
Most Valuable Expert 2013
If the wiring is half-baked, it's quite possible - likely even - that areas of punchdown were done sloppy (network jacks, patch panel) and even cable ends punched down sloppy or even with a wrong type of RJ45 connector (stranded instead of solid or vice versa).  And problems with those can creep up randomly if there's enough vibration from a large truck near by one too many times.  Replace the cables/networking with a CORRECTLY done, PROFESSIONAL job and fully bake this!
Using one pair automatically means you are running half duplex ,it will work ,but much more slowly.

No comment has been added to this question in more than 21 days, so it is now classified as abandoned.

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