Encryption for gateway tunnel-based VPNs independent of wan transport addresses

A source virtual private network (VPN) gateway supports a local source subnet and communicates over a wide area network (WAN) with a destination VPN gateway that supports a local destination subnet. The source VPN gateway receives from the local source subnet an Internet Protocol (IP) packet destined for the local destination subnet, determines a security association (SA) based on a source IP address and a destination IP address of the IP packet, and encapsulates the IP packet with tunnel encapsulation including a tunnel protocol header and a tunnel outer IP header, to produce a clear-text tunnel packet. The source VPN gateway encrypts the IP packet and the tunnel protocol header but not the tunnel outer IP header using an encryption key and a security parameter index for the SA, to produce an encrypted tunnel packet, and tunnels it to the destination VPN gateway over the WAN.