[TriLUG] Any help diagnosing an intermittent network issue?

[Kevin Hunter Kesling]

At 2:27pm -0400 Sat, 25 Oct 2014, R Radford wrote:
> I have my laptop (running Ubuntu 14.04) corrected via wifi to my home
> router - that connect is up and stable.
>
> I then connect the wired network of the laptop to my Raspberry Pi.  I have
> configured both the RPi and my laptop as static on a different subnet.

> However some days the connection between my laptop and the RPi is very
> unstable.  I plug in the Ethernet cable between the two, watch the
> interface (eth0) come up and can connect, but then it drops a few seconds
> later.  Running a watch command on "ifconfig eth0" shows that it toggles
> between up and down every few seconds.

> I don't understand why the interface bounces up and down every few
> seconds (and usually finally gives up) on some days, but with no
> changes works fine another day. I know this will be very difficult to
> debug remotely, so what I am looking for is help on where I should
> look to help diagnose this problem.

Also shooting from the hip, and from the "Is it plugged in?" department, 
are you sure the cable with which you are working is good?  The failure 
mode you describe almost rings the "bad cable" bell in my mind.

Similarly, are you sure that both of the ethernet ports/NICs are sound? 
  Until 2 weeks ago, I thought they roughly never went bad, but a 
Windows dev in a shop I work with told me they "go through them all the 
time."  Okay, I must bite as he swapped out the card in front of me and 
made the errant connection work again.

Two, probably incorrect, suggestions.

Kevin