The modem is a 3Com LAN Modem. A VPN tunnel server exists on the corporate 1720 router. The "real" problem is that the clients using the 3Com LAN Modem are behind the NAT server of the modem. Using NAT and IPSec 3DES, there is no published method known to make a connection.
My experience has been that the 3Com LAN modem will only allow a single VPN tunnel through at a time. I'm not sure if this is the problem that you're experiencing, but you could start with a single pc connected to the LAN modem, bring up a tunnel, and move on from there.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...