When my old company migrated away from sonicwalls we stood up an MX in our HQ, added a few users, added the routes we needed for access and tested. Then went to a site, stood up a MX on the backup circuit as you described and moved a few users over just as we did at HQ. Tested a good 3 weeks and then moved the whole HQ over, then the 1st site. Once we got the bugs worked out, we went site by site swapping the whole sites network out in a day, and tested for a week. Biggest thing we learned was you have to put actual business traffic across the test network. Web browsing is not business traffic (not for us anyway) and then only true test is to do the cut over and see what doesn't work. fortunately we were able to do all the initial testing after hours for HQ and at a small site with little traffic. Hope your project goes well and you enjoy meraki as much much as we did. It really is almost always simple with meraki. The challenges come when you try to integrate 3rd parties
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