This Article explains reason why ZTP is not supported  on FLEX Images for QFX5100

In Non-flex image  when request system zeroize command is issued ,ZTP  is triggered as below and  Auto Image Upgrade log messages are seen 

root>show log messages | match Auto Image Upgrade

Auto Image Upgrade: No DHCP Client in bound state, reset all DHCP clients

Auto Image Upgrade: DHCP INET Client State Reset : irb.0 vme.0 em1.0

Auto Image Upgrade: DHCP INET6 Client State Reset : irb.0 vme.0 em1.0

Auto Image Upgrade: No DHCP Client in bound state, reset all DHCP clients

Auto Image Upgrade: DHCP INET6 Client State Reset : irb.0 vme.0 em1.0

Auto Image Upgrade: DHCP INET6 Client State Reset : irb.0 vme.0 em1.0

Auto Image Upgrade: DHCP INET Client Bound interfaces :

Auto Image Upgrade: DHCP INET Client Unbound interfaces : irb.0 vme.0 em1.0

Auto Image Upgrade: DHCP INET6 Client Bound interfaces :

root> show configuration chassis 
auto-image-upgrade;

In Flex image ZTP  is not triggered as below where Auto Image Upgrade log messages are not seen 

root>show log messages | match Auto Image Upgrade

root> show configuration chassis
root>

• Zero Touch Provisioning (ZTP) using DHCPv4/DHCPV6 options is not  supported on Junos OS Flex images for QFX5100 Platforms alone.
• Because  there is a difference in the factory default configuration between flex and non-flex image in QFX5100 .
• PHC(Phone Home  Configuration )is missing in QFX5100 which is present in other QFX Platforms .Hence ZTP is not supported on Flex Image for QFX5100