[wellylug] Only one of three ethernet cards works at a time

[Jeff Hunt]

I have an odd problem with a friend's computer. It has three ethernet
sockets. One on the motherboard and two cards. I want to set it up
masquerading IPs from a second home computer onto an internet
supplier, which it has done in the past and which I have caused to
happen several times myself on other computers. I installed the latest
Ubuntu and so it had to be set up again but now there is a problem.

Although all three cards work. As soon as a second switch is turned
on, only one card remains active. When I checked in detail the top
socket ran when the second socket was filled. The second socket ran
when the third was filled but the third socket ran when the top socket
was filled. I think the priorities changed after a restart but the
problem stayed the same.
I am at a loss.

I have turned off all preferential routing and firewalls to the best
of my knowledge and I'm using pinging to test the cards. I have tried
pinging from specific cards, but if that card is the idle one then it
stays idle and makes no difference.

[An aside -- If I can't do anything less drastic I will remove one of
the cards and reinstall with just two sockets but I have an uneasy
feeling there was a reason the second card was added to overcome
another problem but I wasn't head honcho then. Does anyone know a
command to add/remove ethernet cards. My experience is that this
doesn't happen naturally and I have had to reinstall the OS just to
get it to acknowledge the arrival or departure of a card.]

Any ideas?