I found that ifup/ifdown eth0 are the exact commands I need to run. I have verified that they work as needed from a root terminal.
I found gnome-schedule in Synaptic which according to description should work the same way. But after installing it and configuring the same commands, I have found that it does not actually run them or something. I see no change.
a couple of hints:
cron jobs can be set up on a
per user basis, on the other side network interfaces could have been configured to not allow to be managed from (non root)
users, you performed the test as
root, but I'm not sure you have set up the jobs as root, just verify about this in control center.
when I use cron, I usually enter the commands using the absolute path (/sbin/ifup, /sbin/ifdown) this will exclude any possible issue PATH related, probably not your issue, but a good advice as general rule when using cron.
from a terminal, running the command,
crontab -l (the option is -L but lowercase), you will get a listing of the scheduled jobs, something like:
0 2 * * * /bin/ifup eth0
0 4 * * * /sbin/ifdown eth0
AS