[ILUG] VPN, ADSL modem - what goes where?

[info at kennedysoftware.ie]

Hello,

Trying to configure a VPN tunnel between 2 SuSE (SLES) servers, eth1 in both 
cases, across ADSL from eircom. It works OK if we have an external 
router/modem box between eth1 and the eircom line. When we replace the 
external box with an ADSL MODEM, and re-config the servers as "Routers" 
(etc?), seems everything still works OK, but we hit VPN config issues... The 
initial "Hi There" packet from the Client never arrives at the VPN server...

Servers:
 - eth0 is internal local LAN
 - eth1 connected to an ADSL "Modem" (eircom)
 - PPPoE, with static IP, from eircom
 - Firewall (SuSEFirewall2) running on eth1
 - "Routes" show ppp0, etc

Conceptually, does anyone know which bit links to which bit in this setup: 
ppp0, dsl0, eth1, firewall... Ie, from inside, are we talking to ppp0, which 
hooks to eth1, etc... or....

I hope the presence of the Firewall should be insignificant, in that we can, 
theoretically, run it or not, without having to change any other "configs". 
Put another way... in the IPSEC file, do we set LEFT to "eth1", or to an 
address we assign to eth1, or to the static IP from eircom (also assigned to 
eth1)... or to ppp0... whew!!

In the IPSEC file, we've been fiddling with interfaces="ipsec0=eth1/ppp0", 
left=, leftsubnet=, leftnexthop=, NAT, etc, but we're now only digging a 
bigger hole!!

Very many thanks for any suggestions or pointers.
   - Mike