I'm wondering if I'm missing something here in the Meraki docs, however, there does not appear to be any way to originate a default route on the LAN side of an MX device. This would then allow the layer 3 switching and routing in the LAN environment to find its way to the Internet via either one of two MX's in diverse DCs. Ideally, the default should only be originated only if a default exists in the MX routing table and is valid (same concept as in IOS with default-originate).
Without the dynamic default route awareness, the LAN layer 3 devices will just blackhole the non-overlay traffic to the primary DC exit device, which may have no upstream comms due to an outage.
I see the MX has the concept of a tracked next hop or host IP via ping to enable upstream routes. Great!
I would implement similar as a workaround on the Meraki MS layer 3 switching, however, tracked static routes does not seem to exist in the MS product set. They can learn a default via OSPF. I just need the means to inject one into OSPF at the Internet edge.
Yeah, I imagine the L2 adjacent MX's in diverse DC's is going to be a hard fit for most/all. The customer would need to stretch every WAN and LAN interface between both DC's. Would result in split brain at some point.
Hopefully, they will add more full-featured OSPF in the future.