[SGVLUG] Bad Ethernet patch cable, but passed a DC cable tester

[Scott Packard]

Had an interesting issue at work with a CAT6 CMR Ethernet cable.  It was an
old cable, CAT6, good quality, field-terminated, about 30 feet, running
directly to a Cisco gig-E switch.  An old cable with a new PC.  It had been
working fine with the old Sun host, until it was retired.  The PC had I/O
issues with it, though a few PC sysadmins had just complained that
something was wrong somewhere, but didn't know where.

(I was using the VMWare's VIC on the PC to deploy an OVF template to a
Fibre-Channel multipathed SAN, so a lot of layers to maybe have problems.)
I figured/looked at some performance graphs in VMWare and thought the
Ethernet I/O should be higher than 3KB/sec, traced the cable back to the
switch and the LED on the switch was quickly flashing from green to yellow
and back.  I replaced the cable with a new one and that fixed the problem.

I wrapped up the old one and brought it home to test it was a DC-based
tester (a Testifier by Test-Um) and all 8 wires pass.  So, I guess
sometimes a DC-based tester won't tell you everything, or, this CMR cable
is stiffer than your typical patch cable and it's just slightly possible
that it may have worked its way loose a little and if I had tried just
remating it to the PC it may have fixed it.  However, it worked well enough
so the PC could authenticate to a domain controller, synchronize time to an
NTP master, and look at a few web-based management sites (ESXi servers).
It just couldn't carry a lot of traffic.

Regards, Scott
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