We currently have MPLS connecting 6 locations. Routing is done by our ISP. Each location has a MX appliance in NAT mode.
We are moving away from MPLS and recently starting to implement Metro-E at our locations. Each location has a designated cheap uplink. Currently I am using each location's core L3 switch to do failover routing for both MPLS/Metro-E and uplink ( I wanted it to use MX auto-vpn if MPLS/Metro-E goes down and if local uplink goes down, send default route to MPLS/Metro-E). Ultimately I want to move all these routes to MX, so I only need to manage local L3 routing on Core switch.
That's how we set it up at one of our branch office. We connected Metro-E on WAN2, then setup traffic shaping to set WAN2 as primary uplink without load balancing, but the according to vpn status, it's forming an auto-vpn with WAN1 instead of WAN2.
WAN2 connection is setup to set gateway to HQ metro-e ip address.
HQ mx has metro-e on LAN port because its WAN1 and WAN2 has separate uplink ISP.