[nzlug] Bridging network cards and speeds

[Daniel Pittman]

Warren Boyd <w.boyd at clear.net.nz> writes:

> It's been a while since I've posted ... 

...and this is a response a little late, but it doesn't seem like all
your questions were answered.

> But I currently have a machine that has two network cards in it, that
> I've bridged to turn two physical networks.

Using the Linux software bridging, presumably.  

> I use X-Windows over the network, I want to move that onto the Gigabit
> network to avoid the lag that comes from slower network speeds.

As others pointed out this probably will not help; Gigabit Ethernet
doesn't actually move the latency of your system much, only the
bandwidth.

> Does anyone know whether the bridging setup will affect the throughput
> to any significant degree (i.e. On one side is 100 Mbit, and the other
> is all Gigabit) - I recall that some setups will lower the speed of
> the network to the lowest speed device.

Yes, it will, in that it requires the kernel to do additional processing
for every packet (enqueue to the other NIC and send).  It can also
effect performance by putting additional memory pressure on the machine
from additional copies (with some NICs) and buffering to cover speed
mismatch.

Linux will not cap the speed of either card, though, so you can still
theoretically get full performance -- an artifact of the bridging being
done at a completely software level.

> Can anyone give me some advice (other than break the bridge - it's
> there as a learning experience currently) on how the bridging will
> affect the speeds?

You will be unlikely to see any significant difference in performance on
either side.

Regards,
        Daniel
-- 
Daniel Pittman <daniel at cybersource.com.au>           Phone: 03 9621 2377
Level 4, 10 Queen St, Melbourne             Web: http://www.cyber.com.au
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