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# Unified ISSU Overview

In software releases numbered lower than Release 6.0, all line modules are reloaded when an SRP switchover occurs. This reload disconnects user sessions and disrupts forwarding through the chassis. Stateful SRP switchover was introduced in JUNOSe Release 6.0 to minimize the impact to the router of a stateful switchover from the active SRP module to the standby SRP module. Stateful SRP switchover (high availability) maintains user sessions during the switchover and data forwarding through the router continues to flow with little impact, thus improving the overall availability of the router.

The unified in-service software upgrade (unified ISSU) feature further extends router availability. Unified ISSU enables you to upgrade the router to a higher-numbered software release without disconnecting user sessions or disrupting forwarding through the chassis.

A conventional software upgrade—one that does not use the unified ISSU process—causes a router-wide outage for all users. Only static configurations (stored on the flash card) are maintained across the upgrade; all dynamic configurations are lost. A conventional upgrade takes 30-40 minutes to complete, with additional time required to bring all users back online.

When you perform a unified in-service software upgrade on a router that has one or more modules that do not support unified ISSU, these modules alone are upgraded by means of the legacy, conventional upgrade process. The unsupported modules undergo a cold reboot at the beginning of the unified ISSU process, and are held down until the in-service software upgrade is completed. Connections that pass through the unsupported modules are lost. The interfaces on these modules pass into a down state, which causes the physical layer and link layer to go down during the in-service software upgrade for those modules.

Applications that do not support unified ISSU applications cannot maintain state and configuration with minimal traffic loss across the upgrade to a higher-numbered release. When you attempt a unified in-service software upgrade on a router on which an ISSU-challenged application is configured, the in-service software upgrade cannot proceed. You must unconfigure the ISSU-challenged application to successfully perform the unified ISSU.

## Router Behavior During a Unified In-Service Software Upgrade

The following behaviors are characteristic of a unified in-service software upgrade.

- Connections that were established before you begin the in-service software upgrade are maintained across the upgrade. Any such connection that was forwarding data continues to do so during and after the upgrade.
- New connections are denied until the upgrade is completed.
- Packet loss during the upgrade is limited. Bandwidth through the modules is reduced, but the impact is minimal.
- Graceful restart protocols do not time out during the in-service software upgrade.
- The unified in-service software upgrade has a minimal effect on the control and data planes. During the SRP module upgrade phase, forwarding through the fabric is interrupted for about 1 second on the E120 and E320 routers and about 4 seconds on the ERX-1440 router. During the line module upgrade phase, forwarding through the chassis is interrupted for about 15 seconds on the E120 and E320 routers and for about 50 seconds on the ERX-1440 router.
- Diagnostic software is not run on any modules during a unified in-service software upgrade.
- The router will undergo a cold restart if you attempt to upgrade the software to a lower-numbered version with unified ISSU. The in-service software upgrade must be to a higher-numbered release than the running release.
- Additional memory is consumed during a unified in-service software upgrade. Available memory on a line module might not be sufficient due to the module’s configuration. Unified ISSU can detect this limitation during the upgrade procedure and exit the process.

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