The design where you have SD-WAN via 1 interface and then use a private WAN solution and route via a LAN port of the MX will work but without SD-WAN features. This means the MX will always route over the LAN port unless that route is no longer available. So you can have a ping probe from the MX that will keep the static route alive. So yes you will having routing via the E-LAN but you want be able to route based on latency, jitter, loss as you would if you would have if you connect the E-LAN to the WAN2 port. So if you can have one E-LAN terminate at a location with a router to the internet to have your VPN registry connection then you could do a true SD-WAN.
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