Juniper QFX5100 - Reinstallation not booting from USB

Due to a software issue on one of our QFX5100 switches, we’ve decided to reinstall the software on the device via the USB reinstallation method. We tried at multiple times to boot the switch from the USB thumb drive without success, even trying multiple different USB thumb drives and different methods to write the installation image onto the switch. The result kept the same: The device showed some cryptic text on the serial interface for a few seconds and then booted once again from the JunOS image residing on the SSDs.

As I ran out of ideas, I checked the file system and found a “syslinux.cfg” file including the options for the GRUB bootloader which seems to be the cryptic text I were able to see on the CLI using macOS and screen within my terminal to connect to the serial device. Prior to editing the options, the file looked similar like this.

PROMPT 0
SERIAL 0
TIMEOUT 100
ONTIMEOUT reboot
DEFAULT Normalboot

UI menu.c32
MENU TITLE Junos Installer - (c) Juniper Networks 2013 

LABEL Normalboot
  MENU LABEL Normal boot from SSD (default)
  LOCALBOOT -1

LABEL Reboot
  SAY Rebooting...
  COM32 reboot.c32

LABEL Install
  MENU LABEL Install Junos
  SAY Starting Junos Installation .......
  KERNEL vmlinuz
  APPEND initrd=initrd root=/dev/ram0 rootfstype=ramfs console=ttyS0,9600n8 intel_iommu=on acpi=ht isolcpus=2,3 libata.force=noncq  install-type=usb clean-install vmdisk=img erase=secure   quiet

LABEL Boot to host shell [debug]
  KERNEL vmlinuz
  APPEND initrd=initrd root=/dev/ram0 rootfstype=ramfs console=ttyS0,9600n8 intel_iommu=on acpi=ht isolcpus=2,3 libata.force=noncq  shell quiet

I swapped the “ONTIMEOUT” option from “Normalboot” to “Install”, replugged the USB thumb drive into the switch. During the next boot of the switch, it automatically booted the JunOS installer correctly formatting the device.