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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 primary role acting as the primary 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 primary VM, and the original primary 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:

  1. The management process (mgd) verifies that non-stop routing (NSR), graceful Routing Engine switchover (GRES), and non-stop bridging (NSB) are enabled.

  2. The switch downloads and validates the software package.

  3. The ISSU state machine spawns the backup Routing Engine (RE) with the newer software.

  4. The ISSU state machine checks to see if the backup RE has synchronized all of the data with the primary RE.

  5. The ISSU state machine moves the devices (for example, forwarding ASIC, FPGA, management port and serial console) from the primary RE to the backup RE.

  6. The primary role is switched between the REs, so the backup RE becomes the primary RE.

  7. The old primary RE is shut down.