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 unified 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 a unified ISSU-challenged application is configured, the unified in-service software upgrade cannot proceed. You must unconfigure the unified 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.

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