On Oct 17, 2007, at 1:20 PM, Jonathan Kunze wrote:

we bring it up using *ip link set eth0 up* 

If you want to use the ifup/ifdown interface in /etc/network/interfaces, then you have to use "ifup eth0" and "ifdown eth0" to bring them up and down.  If you directly call the lower-level iproute2 commands like you're doing, then ifupdown won't start up dhcp clients and such.