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Understanding In-Service Software Upgrade (ISSU)
An in-service software upgrade (ISSU) enables you to upgrade between two different Junos OS releases with minimal disruption on the control plane and with minimal disruption of traffic. During an ISSU, the Junos OS runs in two separate virtual machines (VMs)—one VM is in the master role acting as the master Routing Engine, and the other VM is in the backup role acting as the backup Routing Engine. The Junos OS is upgraded on the backup VM. After a successful software upgrade, the backup VM then becomes the master VM, and the original master VM is no longer needed and is shut down.
ISSU provides the following benefits:
Eliminates network downtime during software image upgrades
Reduces operating costs, while delivering higher service levels
Allows fast implementation of new features
In-Service Software Upgrade Process
When you request an ISSU on a standalone device:
The management process (mgd) verifies that non-stop routing (NSR), graceful Routing Engine switchover (GRES), and non-stop bridging (NSB) are enabled.
The switch downloads and validates the software package.
The ISSU state machine spawns the backup Routing Engine (RE) with the newer software.
The ISSU state machine checks to see if the backup RE has synchronized all of the data with the master RE.
The ISSU state machine moves the devices (for example, forwarding ASIC, FPGA, management port and serial console) from the master RE to the backup RE.
The mastership is switched between the REs, so the backup RE becomes the master RE.
The old master RE is shut down.