# Unified ISSU Phases Overview

The JunosE Software includes software modules that operate the following hardware components:

- SRP module
- Line module control plane
- Line module forwarding plane

A unified in-service software upgrade replaces the currently operating software on each of these components with a higher-numbered release. The unified ISSU also upgrades or re-creates the state and configuration data of the configured applications.

Before you begin the unified in-service software upgrade, you must first prepare the router for the upgrade. See Hardware and Software Requirements Before Beginning a Unified ISSU for more information.

The unified in-service software upgrade takes place in three phases:

1. Initialization Phase—When you issue the issu initialize command, unified ISSU verifies whether all prerequisites for the upgrade have been met. The router is prepared for the upgrade. The configuration that has been mirrored to the standby SRP module is upgraded according to the upgrade release. This phase can last from a few minutes up to 40 minutes depending on the number of software releases across which the router is being upgraded.
2. Upgrade Phase—When you issue the issu start command, unified ISSU again verifies whether all prerequisites for the upgrade have been met. During this phase the line module control plane and forwarding plane are upgraded and all three hardware components are resynchronized.
3. Service Restoration Phase—This phase automatically begins without user intervention when the upgrade phase has completed. During this final phase, the router is returned to a normal, runtime state.

#### Related Documentation

- Unified ISSU Initialization Phase Overview
- Unified ISSU Upgrade Phase Overview
- Unified ISSU Service Restoration Phase Overview
- Halting Unified ISSU During Initialization Phase
- Halting Unified ISSU During Upgrade Phase