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Understanding Automatic Software Update on Virtual Chassis Member Switches

Automatic Software Update Basics

When you have configured automatic software update on a Virtual Chassis, the Junos OS version is updated on the new member switch when you add it to the Virtual Chassis. The new member switch immediately joins the Virtual Chassis configuration and is put in the active state.

For a standalone switch to join an existing Virtual Chassis, it must be running the same version of Junos OS that is running on the Virtual Chassis master. When the master in a Virtual Chassis detects that a new switch has been added to the configuration, it checks the software version on the new switch. If the software version on the new switch is not the same as the version running on the master, the master keeps the new switch in the inactive state. If you have not enabled the automatic software update feature, you have to manually install the correct software version on each prospective member switch as it is added to the Virtual Chassis.

Automatic Software Update Restrictions

You cannot use automatic software update in certain scenarios, and you must ensure that the software release version on the Virtual Chassis is supported by the release on the prospective member switch.

You cannot use the automatic software update feature to update software for a prospective member switch in the following scenarios:

• The Virtual Chassis was preprovisioned and is running Junos OS Release 10.4R2 or earlier.
• You configured the mastership-priority command to manually configure the mastership priority of at least one Virtual Chassis member switch and the Virtual Chassis was running Junos OS Release 10.4R2 or earlier when you committed this configuration.
• The Junos OS versions on the Virtual Chassis and the prospective member switch are different versions of the same major Junos OS release. For instance, if a Virtual Chassis is running Junos OS Release 10.4R1, the prospective member switch cannot be updated using automatic software update if it is running Junos OS Release 10.4R2, 10.4R3, or any other Junos OS Release 10.4 release version.

The automatic software update feature also has a Junos OS release dependency between the release that is already running on the Virtual Chassis and the release that is running on the prospective member switch.

Table 1 summarizes automatic software update support for each Junos OS release combination.