This article helps with the command on activating USB mini port as console interface on EX3400.

By default, Juniper switches prioritize the RJ-45 console port. To switch this permanently to the Mini-USB port you need to activate it by setting the below command, this ensures the USB-MINI port is activated and remains so after reboots, provided the configuration is not lost.  

user@switch# set system ports auxiliary port-type mini-usb

After configuring, commit the changes and reboot the switch:

user@switch#commit

user@switch#reboot

This sets the Mini-USB Type-B console port as the active console port, allowing you to connect to the switch using a Mini-USB cable. The mini-USB console port is just an alternate console interface to the RJ-45 console.

Important Considerations:

• Ensure the USB to Serial driver is installed on the host machine.

      • The default HyperTerminal properties should be set as follows: Baud rate—9600, Flow control—None, Data—8, Parity—None, Stop bits—1, DCD state—Disregard.

Additionally, if configuration loss is a concern, ensure proper configuration management practices, such as saving the configuration to persistent storage or avoiding factory resets without backups. Enabling rescue configuration serves as a fail-safe that the switch can roll back to automatically:  

> request system configuration rescue save