We have couple of EX4200s switches in Virtual Chassis mode running in the network. Each Virtual chassis has only 2 member switches.

We are required to upgrade the Junos OS on each member switch in the virtual chassis.

There are two approaches to perform Junos OS upgrade.

1) Perform the Junos OS upgrade on all members of the virtual chassis at the same time. This will be bring complete outage for period of time till all member switches come back after reboot.

2) Perform Junos OS upgrade on each member switch individually instead of whole virtual chassis. This however will avoid complete service outage.

Which option is considered to be the best practice / recommended approach that has a high probability of success and less prone to instabilities / abnormal behavior post upgrade situation.

If some one could help on this , would highly appreciate that.

Many Thanks.

  • You might want to look at NSSU Aug 21, 2019 at 8:23
  • @Andrey ProKhorov. We havnt tried NSSU before even in production environment and may be never use in near future.
    – Nabeel
    Aug 21, 2019 at 8:49
  • @NABELL NASIR considering your question, best option will be - #3 Use NSSU. If you're really limited to those two options, you can go with variant two. But it will still cause traffic disruption. Aug 21, 2019 at 14:46
  • @ Andrey Prokhorov. You are right abt option 2 . It will cause degradation even if we enable GRES NSR and NSB. My point is what's the best approach that may cause outage but is smooth and bug free after upgrade. I saw video on YouTube from Juniper networks and they showed to upgrade junos on EX4200 VC using option 1.
    – Nabeel
    Aug 21, 2019 at 14:50
  • yeah, but that means your VC will be unable to forward data while system will reboot/upgrade. And it's approx ~5-7 mins of downtime. Aug 21, 2019 at 14:59

1 Answer 1


As per discussion on various forums and with Juniper networks , recommendation will be upgrading the whole virtual chassis as it has less probabilities to fail and it is simpler. The process should take around 10-15mins.

Doing it one by one will take longer, and it has more chances to fail when the units are split. Please keep in mind that while they are split only one of them will be forwarding traffic.

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