I did the HA method to replace a MX400 from an RMA. However I had to swap back and forth a couple of times as the RMA did not send me the SFP+ module and the modules are NOT hot-swap. But it worked. Each time the network stopped for only about 10 seconds. The one issue we had that I had not seen mentioned anywhere is on the MS switch settings, I had the old MX mac address as the only one allowed for DHCP. You need to add both mac addresses (old and new MX) to this setting before starting the swap in order to not cause DHCP blockage. I did not, and to make things worse, I initiated a MS firmware upgrade which meant that when the MS did a restart it did not get its address from the new MX and could therefore not reach the cloud to update even the DHCP mac allowed setting. I had to physically visit every switch and give it a static - once connected to the cloud, the new allowed DHCP mac address synced and then everyone could get back into the network. Simple mistake, bad consequences. Just add to your list of things to check.
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Rarely I have seen issue with shim drivers, as installed by antivirus or other VPN clients. Try uninstalling anything that looks like it could install something into the network stack. If it works slowly reinstall the bits of software one at a time. Sometimes the order that shim drivers get install is important. Failing that, try seeing if there are any NIC driver updates (Ethernet and WiFi).
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