This is a Resolution Guide, which steps you through the method to check the compatibility of QFX5100 switches in virtual chassis with other EX/QFX series switches.

This article helps you to understand mixed mode and non-mixed mode supportability in VC with QFX5100.

Starting in Junos OS Release 13.2X51-D20, QFX5100 switches support Virtual Chassis. You can interconnect up to ten switches into a QFX5100 Virtual Chassis that can include any combination of QFX5100, QFX3500, QFX3600, and EX4300 switches.

QFX5120 and QFX5100 not compatible in VC.

https://www.juniper.net/documentation/us/en/software/junos/virtual-chassis-qfx/topics/concept/virtual-chassis-qfx-series-understanding.html#understanding-qfx-series-virtual-chassis__d4645e358

QFX5120 and EX4650 switches are similar and operate the same way in a Virtual Chassis. Each QFX5120 or EX4650 switch model can only be combined with the same model of switches into a Virtual Chassis.

QFX5120 switches cannot be mixed with any other models of switches in a Virtual Chassis.

https://www.juniper.net/documentation/us/en/software/junos/virtual-chassis-qfx/topics/concept/virtual-chassis-qfx-series-understanding.html#understanding-qfx-series-virtual-chassis__d4645e239