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

[Daniel Pittman]

On Fri, Jun 10, 2011 at 17:26, Jeff Hunt <jeffhunt90 at gmail.com> wrote:

> 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 you say that the second switch was turned on, or the second
socket connected, do you mean that they were connected to completely
physically disconnected equipment, or were the sockets on the system
connected to the same broadcast domain?  (eg: to the same switch, or
to two switches that also had a cable between them, or something like
that.)

Linux uses the "weak end host model", in which the IP address is a
property of the *system*, not the network port, even though it looks
like the later.

That means that if you plugged two cables into a single broadcast
domain, Linux will pick *one* of them to use for all communication,
out of the box.  Which might explain what you saw: the second port
was, essentially, ignored even though it "worked", and the preference
was quasi-random.

Daniel
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