[gentoo-user] Net problems

Hi,

I have a problem with some kind of a lokal net connection (ethernet
over USB):

Current handling is as follows:
I boot my Gentoo-Linux PC, which connects to my Fritz!Box, collect 
EMail etc...works fine so far.
Then I connect an embedded system (Arietta G25
www.acmesystems.it/arietta) to the usb port and wait until this
little thing has booted.
On my Linux PC I do the following then (as root):

ifconfig usb0 192.168.10.1 up
echo 1 >! /proc/sys/net/ipv4/ip_forward
iptables -A FORWARD -i eth1 -j ACCEPT
modprobe ip_tables
modprobe ip_conntrack
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
ssh -t root@ariettaa abduco -A root

(I real life I have two Ariettas which I connect to the
PC the same time with two different IP-addresses and two
different USB-ports)

This connects me finally to the Arietta, which do ethernet over USB.
I put the commands into a script, so not too much hassle here.

But!
On the Arietta I also installed Gentoo. An update, especially
when it includes gcc can take up to three days (I tried
crosscompilation and some other construction and I am currently
most happiest (sounds a litte too much like 'german english'...;)
with this).
So I first fetch all needed files and then restart the update
and detach from the terminal.

Now the problem:
When I shutdown the PC, which needs not to run all the time, and
restart it later again I am no longer able to connect to the Arietta
with the steps described above. All needed modules are loaded while
booting the PC and ifplugd is installed on the Arietta.

Unfortunately I have not found any differences of the USB interface,
which would make it possible to handle the Ariettas via udev.

How can I make a reconnect after rebooting my PC possible every time?

Thank you very much in advance for any help!
Best regards,
Meino