Question:You are configuring the final elements of a migration effort where resources have been moved...

Validate that the Compute Engine instances are using the Metadata Service IP address as their resolver. Configure an outbound forwarding zone for the on-premises domain pointing to the on-premises DNS server. Configure Cloud Router to advertise the Cloud DNS proxy range to the on-premises network.

Validate that there is network connectivity to the on-premises environment and that the Compute Engine instances can reach other on-premises resources. If errors persist, remove the VPC Network Peerings and recreate the peerings after validating the routes.

Review the existing Cloud DNS zones, and validate that there is a route in the VPC directing traffic destined to the IP address of the DNS servers. Recreate the existing DNS forwarding zones to forward all queries to the on-premises DNS servers.

Ensure that the operating systems of the Compute Engine instances are configured to send DNS queries to the on-premises DNS servers directly.