This article explains the if In-Service Software Reboot (ISSR) is supported on Juniper Networks [EX/QFX] Virtual Chassis applications to assure minimal traffic disruption.

Customer is asking if In-Service Software Reboot (ISSR) is supported on Virtual Chassis stack to assure minimal traffic disruption, as Juniper documentation only presents this feature on standalone switches, but it does not mention if it is supported on virtual chassis applications or not.

Even if you have all necessary configuration such as Non-stop routing (NSR) and graceful Routing Engine switchover (GRES), virtual chassis applications will not perform In-Service Software Reboot (ISSR), and you will get an error saying "error: System not ready for ISSR.". Below is an example of what this would look like:

Configuration added and committed prior to ISSR attempt:

set chassis redundancy graceful-switchover
set routing-options nonstop-routing
set protocols layer2-control nonstop-bridging
set system commit synchronize

Results:

{master:0}
user@switch> request system reboot in-service
Reboot the system ? [yes,no] (no) yes
[Sep 29 15:58:04]:ISSU: Validating Image

PRE ISSR CHECK:
---------------
PFE Status              : Online
Member Id zero            : Valid
VC not in mixed or fabric mode    : Valid
Member not in single node vc     : Invalid
BFD minimum-interval check done    : Valid
GRES enabled             : Valid
NSR enabled              : Valid
drop-all-tcp not configured      : Valid
Ready for ISSR            : Valid

error: System not ready for ISSR.
error: aborting ISSR
[Sep 29 15:58:06]:ISSU: IDLE

{master:0}
user@switch>

https://www.juniper.net/documentation/us/en/software/junos/high-availability/topics/task/in-service-software-reboot.html