Re: Multiple Ethernet Cards

Subject: Re: Multiple Ethernet Cards
From: Peter M. Bagnall (
Date: Mon Dec 06 1999 - 12:49:12 MST

>If I turn on eth0, plug the built in card into my hub, and assign it
>a different IP address than my eth1 card, what will happen? Will I
>really be using both cards for more throughput? Could I do this
>without giving the second card a different IP address?

If you plug both cards into the same hub my guess would be that you
throughput would, if anything, drop. The reason being that if the hub is a
100Mb (which I'm assuming), then it can't transfer more data that you can
pump out through your 100Mb card anyhow. I'd guess (and it is just a
guess), that the only thing you'd do by using both cards in increase the
chance on packet collisions, causing both cards to randomly backoff (which
is what ethernet does when it detects collisions). Can you actually
generate enough *real* traffic to saturate your 100Mb link anyway? I would
have thought that other things would give out before that. But I'd be
interested to be proved wrong.

The way I've heard about multiple interfaces being used usefully is to give
them different IP addresses, but it only really makes sense if you have a
switched network architecture and seperate cables from each interface to
the switch (that way avoiding the shared medium). If you do that then you
will improve throughput. I've seen a machine get so overloaded on it's only
interface that the machine could no longer do DNS lookups. It was getting
thousands of requests for 20Mb files via HTTP one day and couldn't cope. If
it had had another interface it could have used for everything other than
the webserver it would probably have performed much much better. (The
problem was the webserver was using all the bandwidth so the machines other
services couldn't do anything, and since the machine was primarily a
fileserver it became useless, an interface dedicated to the webserver would
have solved the problem).

Also, I'm not sure about this but some hubs may degrade to 10Mb if you plug
in both 100s and 10s in. Could someone confirm of deny that, I'm really not

Does that make sense?

To be honest though, unless you have a problem like the webserver example,
I'd not bother. You'll probably not get much out of it.


Peter M. Bagnall -

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