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Understanding Software Upgrades in a Virtual Chassis Fabric

This topic provides an overview of software upgrades on Virtual Chassis Fabric (VCF).

Virtual Chassis Fabric Software Basics

VCF is initially supported in Junos OS Release 13.2X51-D20. All devices in a VCF must be running the same version of Junos OS that supports VCF.

At initial VCF configuration, you should update all devices to the same Junos OS release before inteconnecting them into a VCF. Also, as a best practice when you add a device to an existing VCF, you should update the Junos OS release on the new device to the Junos OS release running in the VCF before interconnecting it into the VCF. Adding a new member that already has a compatible software version to an existing VCF ensures that the VCF absorbs the new member gracefully without the downtime required to reboot the device after an automatic software update or needing to troubleshoot issues adding the device due to mismatched software releases.

Nonstop Software Upgrade (NSSU)

With nonstop software upgrade (NSSU), you can upgrade the software running on all member devices in a VCF with minimal network traffic disruption during the upgrade. NSSU upgrades the software on each device in the VCF while the other devices not being upgraded can continue to pass traffic and perform normal operations.

NSSU is supported in a QFX5100 VCF.


You can’t use NSSU to upgrade a QFX5100 VCF from a Junos OS “-qfx-5-” image (see the package filename) to a “-qfx-5e-” image. You must first upgrade all of the QFX5100 switches to a “-qfx-5e-” image that supports NSSU. Then you can use NSSU to upgrade the VCF to a later “-qfx-5e-” Junos OS release for supported from and to release combinations. See Upgrading a QFX5100 Switch with a USB Device to Join a QFX5110 Virtual Chassis or Virtual Chassis Fabric.

NSSU includes a line-card upgrade group feature that you can optionally configure on a QFX5100 VCF to upgrade multiple linecard role member switches in parallel, which reduces the total time needed to complete an NSSU operation. NSSU upgrades the VCF linecard role members in the order in which you define the upgrade groups, so you can also use upgrade groups to control the upgrade sequence. You can also run NSSU without defining upgrade groups.

For additional information on NSSU in a VCF, see Understanding Nonstop Software Upgrade on a Virtual Chassis Fabric.

For more information on configuring upgrade groups, see Configuring Line-Card Upgrade Groups for Nonstop Software Upgrade.

Automatic Software Update

If you configure the automatic software update option in a VCF, when you add new devices to the VCF, the VCF automatically updates the Junos OS on the new members to the version of Junos OS running on the VCF when you cable the new devices into the VCF.

You enable automatic software updates using the set virtual-chassis auto-sw-update CLI command.

See Upgrading Virtual Chassis Fabric Software Using Automatic or Standard Software Update Features.


You must first upgrade a QFX5100 switch running a Junos OS software image with “-qfx-5-” in the package filename to a Junos OS software image with “-qfx-5e-” in the package filename before you can add it to a QFX5110 Virtual Chassis or VCF. The automatic software update process cannot automatically update a switch from a “-qfx-5-” image to a “-qfx-5e-” image. See Upgrading a QFX5100 Switch with a USB Device to Join a QFX5110 Virtual Chassis or Virtual Chassis Fabric.

After you install a QFX5100 switch with a “-qfx-5e-” Junos OS software image, the automatic software update process can successfully update the switch automatically with a different version of a “-qfx-5e-” Junos OS image to match the other members in the Virtual Chassis or VCF.

Standard Software Upgrade

To upgrade software on a VCF using the traditional update method for Junos OS, you log onto the primary Routing Engine and use one of the following CLI commands:

  • To initiate the upgrade on a non-mixed VCF: request system software add

  • To initiate the upgrade on a mixed VCF where you need to specify multiple packages for the different types of switches in the VCF: request system software add set [package-name package-name ...], where each package-name is the path to a different image for each device family in the mixed VCF.

Upgrading Junos OS on a VCF using the standard software upgrade commands disrupts traffic flow in the VCF. The process brings the entire VCF down until the upgrade completes and then brings the VCF up again.