[lug] 2 NICs Same subnet

rm at mamma.varadinet.de rm at mamma.varadinet.de
Wed May 9 03:23:52 MDT 2001


On Tue, May 08, 2001 at 08:11:59PM -0700, Stephen Smith wrote:
> I do this now and see a few performance problems with
> it. Is why I am asking for other options.

How did you verify that the performance problems are caused
by the aliasing? That would imply that the bottleneck is actually
the NIC, which is kind of hard to belive. Or do you intend
to have each NIC attached to a cable that connects to a switch?
Otherwise the cable is most likely the limiting factor.

I've used two NICs from the same manufacturer several times
without major problems so far. With some NICs you need to set
the IO/IRQ params for the individual card (the exact parameters
can be found in /usr/src/linux/Documentation/networking/..)
If you compile the driver into the kernel you need to set the
params at boot time (man lilo.conf will help you), if you complie
them as modules you could give the params to insmod.

> > Also bear in mind that if they're both on the same
> > subnet, you *could*
> > just use IP aliasing.

Aliasing also works with addresses on different subnets,
AFAIK.

You'll also need to set up some custom routing 
/firewalling to mke it clear to the kernel on what
interface packets should _leave_ the box. Otherwise
you machine will most likely send out all packets on
one interface. 
BTW, _iff_ you decide to to this way, have a look at
'Policy Routing using Linux' by Matthew G. Marsh.
Small and not cheap but worth every penny. Also have
a look at the 'iproute' command that gives you much
more control over interfaces/routing than the old
ifconfig/route commands (the book gives a good intro-
duction into this, that alone is worth the buy).

 Ralf



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