The SD-Wan runs in a "out-of-band management" interface. That is a pre-configured VLAN that will try to get a DHCP address. This is configured on the "Switch Settings" sections of the "Switch" tab of the Meraki Dashboard.  If this interface doesn't get Internet connection, it will try to go via any of the trunked VLANs configured in the switch. The switch will try any method to reach the Meraki DCs. Therefore, you need to make sure that this Management VLAN has internet access, but the rest is all layer 2. ... View more

If the 425's are not going to provide Layer 3 services, the design is pretty simple, as the other friend explained. But then, will you have a single Layer 3 SD-WAN equipment? In that case, you are keeping the Single Point of Failure anyway, so the 425s are not really a good idea. You might achieve the same results with 355s or even lower if you don't need the 40Gb ports.  ... View more

Hello Mac1 You need to create a port channeling between all the stacks. Let's say that the 425s are the core stack, the L2 switches 1 and 2 are stack 1, and L2 3 and 4 are stack 2. You will have a port channeling between core stack-stack 1 and core stack-stack 2. It will be treated as a single switch with 2 different switches down.  You could enable Spanning Tree in that case for a failover if the port-channeling breaks for some reason, but that's a bit risky. ... View more