Loopback for Cat5 works on older NICs but not a newer Gigabit NIC?

[DaveLembke]

Friend of mine just showed me a strange problem he found. His new computer has a gigabit NIC and the NIC works flawless when connected to the router. When plugging in a loopback to the Realtek Gigabit NIC it wont link. When using this same loopback on another NIC on another computer it links with no problem.

He made a new loopback and tested it on the older computer with 100mb NIC and it tested fine and then he inserted it into the Gigabit NIC of the new system and once again no uplink, yet you can connect a network cable to this NIC and it will link and work then.

Anyone know why the loopback doesnt work with this NIC? Is there something different with Gigabit NICs to where loopbacks wont work.

I was thinking that maybe it was an issue with full duplex vs half duplex, but its not configured for full duplex...   

[Base10]

Hey there,

Sorry not had enough time to get a good read at this, but hope this link works for you 

http://www.juniper.net/techpubs/en_US/junos14.1/topics/topic-map/ethernet-fast-and-gigabit-loopback-testing.html

[DaveLembke]

Hello Base10... thanks for providing link... I found this same link the other day on google search and it didnt point out anything unfortunately to specifically test for in relation to why this NIC has no problems with a Cat5 cable connection to a switch or router, but yet with a loopback that works perfectly fine in other computers as tested, it doesnt work on his system which is very strange. The only difference is that this network adapter in his computer is 1000mbps ( gigabit ) and with loopback tested against my systems which are 100mbps.

[strollin]

Newer NICs (Gigabit type NICs) implement Auto Crossover which automatically configures the NIC to cross the TX and RCV lines when needed.   This may be defeating the loopback.  I would try a cable that connects TX to TX and RCV to RCV instead of crossing them.  http://www.transition.com/pshelp/cross.html

Did some more reading.  For a 10/100 NIC it requires 1 set of TX & RCV lines to be crossed, for a 1000 NIC it requires 2 sets of TX & RCV lines to be crossed.  When your friend made the new cable, did he cross 1 pr or 2? 

[DaveLembke]

Thanks strollin .... I made him up a crimped RJ45 with the loopback pinouts that you shared and that did the trick!!!! SWEET!!  Learned something new through this which was unexpected with that a special loopback is needed for gigabit.

Here is the loopbacks I have used with no problem on the 10/100 NICs that didnt work with the Gigabit ( 1000 ) connection.



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