I have successfully established an IPsec tunnel between my mono box
and a Cisco VPN concentrator, however it's not passing any traffic. 
When I try and ping from the mono box through the tunnel, I'm getting
an "icmp unreach/admin_prohibit" error message in my syslog on the
mono box.

I've attached a text file w/ the full log message and a quick drawing
of what the network situation looks like.  The cisco router is doing
NAT w/ ports forwarded to the VPN box behind it.  I think that's where
the problem lies.

Does anyone have any ideas here?  When I check the traffic stats of
the tunnel from the vpn box, it shows that it has transmitted several
hundred bytes through the tunnel, but has received zero bytes.  If I
try and ping from the cisco vpn box through the tunnel, I can see the
Tx byte count go up, but nothing in the Rx count.

I hope I've provided enough information here to give you all a clue as
to how things are set up....

Thanks so much!
-Erik