Hi,
If you are using the S1720 as the access switch, you are recommended to check the STP configuration on the device. Whether the STP function is enabled on S1720, and the interfaces that connect to the host terminals are not set to edge port. If yes, it will take about 30 seconds while you start the connected devices. You can disable the STP function on the switch, this is not recommended, or set these ports to edge port so that they could forward the traffic as soon as the interface comes up.