SD-WAN policies can be configured to control and modify the flows for specific VPN traffic. With multiple WAN uplinks, the MX will proactively build multiple tunnels with each available WAN interface. In the case where there are redundant WAN connections on the security appliance, traffic flows based on the type of traffic traversing the VPN connections can also be configured to allow for best performance. Custom policies set to desired preferences can be set to ensure traffic flows take the appropriate path based on your environment. If a WAN connection that normally handles traffic such as file transfers begins to have performance issues, the Cisco Meraki MX can dynamically change the VPN connection to an alternative WAN uplink. This is done with custom policies or predetermined policies on the dashboard. It is encouraged to configure said policies in your deployment to best fit the needs based on the nature of the traffic and the capabilities of the WAN connections available on the MX.
If you do have questions about what policies are best for your deployment, you can always reach out to either a Meraki Sales Engineer or your Meraki partner for a consultation on what best fits your needs.