Which VPN ID is used for the WAN Edge transport interface connecting to the underlay/WAN?

VPN 0 is the correct answer because it is specifically reserved for the transport VPN, which handles all WAN-facing interfaces and routes to the underlay network. This VPN carries encrypted overlay traffic and management traffic between SD-WAN devices. VPN 1 is typically used for data plane services within the overlay network, often for guest or specific user traffic. VPN 512 and VPN 511 are also used for data plane services and are part of the range of VPNs (1-511 and 513-65534) available for user-defined service VPNs, but they are not designated for the transport interface itself.