Thank you for the explanation, right now, I can stay connect to my Uni-Cisco VPN using both cpngui or --idle things, but one problem that occur to me is I can connect to skype or anything that I have installed in my N900 such as a forecast or go to maemo to install a program, but I can't browse to web, or even download from maemo.org..
Is there any step that missed by me? so I can't browsing to internet?

Here is when I use the --idle=0...
sudo vpnc --dpd-idle=0 my_uni
route : SIOCDELRT : no such process
route : SIOCDELRT : no such process
sending state in dbus : connected
vpnc started in background (Pid : 1765)...

is there any mistake?