Unnumbered GRE tunnel

Hi All!
I try to configure unnumbered GRE tunnel, and I can't find any
tutorial or howto, or even example to configure it.
The topology is one tunnel between two linux boxes:
-- eth1-|__|-eth0 <-------------> eth0-|__|-eth1 --

I'm trying now with:
ip tu add tun1 mode gre local loc.IP remote rem.IP ttl 255 dev eth0
ip addr add tun1 0.0.0.0
ip link set tun1 up

With "ifconfig" I get "no such device" or "can't assign the requested
address", so I use iproute2
But traffic doesn't go over the link. Tunnel appears as UP.
Have somebody configured such thing before? I'm asking for any clue
for this issue.
LARTC mail group is silent..
Thanks in advance!
--
WBR Sagi Sh