Which tunnel type is used to connect one WAN Edge router to other WAN Edge routers?

IPsec is the correct tunnel type because Cisco SD-WAN uses IPsec for establishing secure data plane tunnels between WAN Edge routers. These tunnels encrypt and authenticate traffic, ensuring secure communication across the overlay network. DTLS (Datagram Transport Layer Security) is primarily used for control plane connections between WAN Edge routers and vManage, vSmart, or vBond controllers, not for data plane tunnels between WAN Edge routers themselves. SSL VPN and TLS (Transport Layer Security) are generally used for remote access or secure web communication, respectively, and are not the standard tunnel types for inter-WAN Edge router data plane connectivity in Cisco SD-WAN.