VPN 0 is the transport VPN in Cisco SD-WAN. It carries control traffic and provides connectivity from WAN Edge routers toward the transport networks, such as MPLS, Internet, LTE, or other WAN services. Control connections to Cisco SD-WAN controllers are established through transport interfaces that belong to VPN 0. Data VPNs carry service-side user traffic and are separate from the transport VPN. VPN 512 is commonly used for out-of-band management, not for carrying overlay control connections. VPN 128 and VPN 256 are not the reserved transport VPN identifiers. The design importance is that VPN 0 must have correct addressing, color assignment, routing reachability, NAT traversal considerations, and certificate-based controller connectivity. If VPN 0 is misdesigned, the WAN Edge cannot establish stable control connections, cannot receive OMP routes and policies, and cannot build the secure overlay properly. Therefore, VPN 0 is the correct transport VPN for underlay control traffic. Reference topics: Cisco SD-WAN VPN 0, transport VPN, control connections, WAN Edge onboarding, overlay formation.
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