When setting up Non-Meraki VPN connections between two MXs in different organizations, make sure to populate the Remote ID field of the Non-Meraki VPN peer with the private IP address of the remote MX if all of the following conditions are met:
The MXs are running firmware version MX 15 or higher.
They do not use a User FQDN.
They are connected behind an upstream NAT device.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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