My company is now using a frame relay network as the corporate WAN. As the company expands, the WAN needs to cover more and more locations where frame relay is too expensive. So we start to use a PIX-to-PIX VPN solution to connect new site together. What make me worried about is that because the new site is having their own local ISP and we cannot control the internet access of those new sites, there is a possibility that hacker can go to our frame relay network through the VPN connection.
What is the most effective way to secure a mixed WAN.