How does ethtool work? Why are effects of changes slow?

When using ethtool I initially use the command: ethtool -s eth0
autoneg off duplex half speed 10 to set my network card for a test I am
about to perform. What I do is download a file off the web server on
the computer that just had it's link rate set to 10Mb/s. Two seconds
into the transfer I change the link rate to 100Mb/s with the command:
ethtool -s eth0 speed 100 it works fine I just have a question about
the delay.

When timing the light changes on my 10/100 switch it is approximately 1
second during the transition from 10Mb/s to 100Mb/s. However, with
timings done during the file transfer and verified with
wireshark/ethereal traces the actual delay is 4-6 seconds. I have been
looking for information on this for several days and have not been able
to come up with anything. I was not able to gather any new information
from the trace logs either and was curious if anyone had any input on
this delay. I am working on this for a project at my school and I need
to minimize the delay in the change of speed as much as possible. Do
you have any comments, suggestions or references that would help me?

One more question: How does ethtool work? Does it reset the NIC? Unload
and load the driver?

Thank you for your time,

Matt

On a new note:

It appears that it may not be a ethtool problem. When doing the same
test, but instead of changing the link speed I actually unplug from the
network for 1 second, I experience the same 4-6 second delay.

I am thinking it may have more to do with TCP and the lower layers than
with ethtool.

Any comments?

-Matt

matt wrote:
> On a new note:
>
> It appears that it may not be a ethtool problem. When doing the same
> test, but instead of changing the link speed I actually unplug from the
> network for 1 second, I experience the same 4-6 second delay.
>
> I am thinking it may have more to do with TCP and the lower layers than
> with ethtool.
>
> Any comments?
>
> -Matt

Depending upon what your computer is connected to, this may be loop
detection. When you plug an ethernet connection into a switch, some
switches take a few seconds to make sure you haven't created a loop.
Routing live packets onto a loop could cause the switch to saturate as
each packet goes in one port, out one or more other ports, back in some
other port, and round and round.

So the switch may send a few test probe packets, look to see if it gets
those same packets back on any of its ports, and if so, shut the port
down. (Or, if goes through another switch that's running spanning tree,
negotiate to use that port and shut down a different one.)

DS