I need to have a site to site VPN between two sites.
Here is the following topology for each site:
Site A: One Cisco 1921 WAN port (192.168.3.2) connected to ISP router (192.168.3.66), both the Cisco 1921 and the ISP's router are doing NAT Overload.
Site B: One Cisco 1921 WAN port (192.168.2.2) connected to the ISP router (192.168.2.66), both the Cisco 1921 and the ISP's router are doing NAT Overload.
From the above topology it is clear that I do not have control over the ISP router to do port forwarding.
How can I successfully configure a Site-to-Site IPSec tunnel between the two routers?
I am not asking about "How to do the VPN in Cisco IOS", I know how to do it. But I am wondering, how is it possible to make a VPN tunnel over such double NATing considering that port forwarding is not possible on the ISP router?