SD-WAN with layer 2 multipoint metro ethernet

[tgold]

I'm an sd-wan newbie so bear with me. We have 3 sites with Fortigates that each have their own internet connection as well as a metro ethernet connection that basically acts like a virtual switch meaning that they all have a layer 2 connection to each other over a single ethernet interface at each site. I set them up on the same subnet and they can all reach each other and pass traffic. I have ipsec tunnels that are all working as well.

I've messed around with a simple link monitor before, but I'm wondering how I can setup sd-wan to automatically fail over the connection between sites. It would be nice to have the monitoring that sd-wan provides on the quality of the link. When I try to add the metro ethernet connection, it needs a gateway and I'm not sure what to put here since there are multiple sites it can reach and there is no ISP gateway. Is there a way to specify a different gateway based on the destination subnet? Any ideas or links to tutorials. I've done a ton of searching but can't find any simple answers.

[lobstercreed]

I'm doing a similar thing but with a hub and spoke topology.  I created a subnet for each site's link to the hub so I'm using that link's hub IP as the gateway.  I'm not sure this is the most recommended way of doing this as this does not offer ADVPN shortcuts or failover to a secondary hub, but it was a way that I could get my head around it, so I went for it.