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.
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