If two servers checkbox is not ticked:

Data packets are being sent to IP 1, if for some reason the data fails to be sent to IP 1 and the ACK from server is not received, then the device starts sending data only to IP 2 forever if IP 2 sends ACK, unless it is restarted. if server 2 is also not responding device will try to connect to IP 1 again.
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If two servers checkbox is ticked:

Data packets are being sent to both servers. But the device only guarantees delivery to the IP 1. It will only resend the data to first server if it will not receive ACK from it. If it will not receive ACK from IP 2, the data packet sending will not be repeated to IP 2.

Only records are sent to the server with IP2. Other data packets, such as Transparent Channel, Tachograph, SD card, Garmin will not be sent to the second server.
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