I've attached a "good" debug and a "bad" debug. The "good" debug is when the 871 is plugged into a DHCP cable connection. VPN works perfectly fine when I use this broadband cable connection. However, when I have the 871 plugged into a Top Global cellular router which is NAT'd and port forwarded, the VPN will not establish. This is what the "bad" debug contains. The Top Global cellular router will assign a DHCP NAT IP of 2.2.2.2/29 to the WAN interface of the 871.

I know as a fact the NAT and port forwarding configuration on the Top Global cellular router is working properly because I can establish a VPN connection when I'm using a Juniper SSG5 device. The concentrator for this Juniper SSG5 is a Juniper ISG2000.

Both the Cisco 3845 concentrator and 871 are running IOS 12.4.

I don't see commands available on the 871 to enable NAT Traversal.

To sum things up:

- Configuration on the celluar router is correct because Juniper devices can establish a VPN connection

- VPN will only establish when Cisco 871 is plugged into a non NAT'd DHCP broadband connection

Any tips or suggestions are greatly appreciated, thanks in advance.