iptables for IPSec (Cisco VPN) - where can I find a sample configuration?

I have been unsuccessful in finding an iptables configuration that
allows my Windows 2000 workstation to establish a VPN connection
through my Linux firewall.

Someone please share his/her working configuration for iptables with
me. I'd appreciate that very much!


My setup is:
Linux firewall (RedHat 7.2) with iptables, connecting over ADSL
The Windows workstation is running Cisco's VPN
I have tried many different ways to get this to work, including
allowing udp and ip 50 traffic. However I fail to ever get beyond the
first isakamp handshake. When doing a tcpdump on ppp0 while attempting
a connection, I get nothing more than the following before the VPN
client gives up:
# tcpdump -n -t -i ppp0
<fw_outside_IP>.isakmp > <VPN_gw_IP>.isakmp: isakmp: phase 1 I agg:
[|sa]
<fw_outside_IP>.isakmp > <VPN_gw_IP>.isakmp: isakmp: phase 1 I agg:
[|sa]
<fw_outside_IP>.isakmp > <VPN_gw_IP>.isakmp: isakmp: phase 1 I agg:
[|sa]
where
"fw" stands for the Linux firewall, and
"gw" stands for the gateway on the other end (corporate Cisco
concentrator).


Thanks a lot.
--Ulf