A plant's private interconnect into Google Cloud has failed. The backup is a cellular internet circuit with IKEv2 to Cloud VPN. There is no SD-WAN overlay. What is that backup?
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Short Explanation
The private road is down, and the spare is a public one with a lock on it. You are on native IPsec to Cloud VPN over cellular. The radio does not extend the interconnect, and there is no SD-WAN overlay on this backup.
Full Explanation
Cloud VPN over the internet is native IPsec: the underlay is public, the endpoint belongs to Google, and the payload is encrypted. Cellular is just the internet circuit. It does not extend the failed private interconnect, and it does not become a cross-connect because a carrier operates the tower. Catalyst SD-WAN internet connectivity needs the controllers and OMP. The plant has neither on this backup. Exam caveat: the backup only helps if Cloud Router, or the static routes, prefer it when the interconnect drops. A tunnel that is up and not in the routing table does not carry the failure. Operational check: while the interconnect is shut, the plant should show an IPsec SA to the Cloud VPN outside address and the VPC prefixes through that tunnel.