# Halt of Unified ISSU During Upgrade Phase Overview

During the upgrade phase—before the line module and control plane software is upgraded—the unified ISSU process provides an opportunity to cancel the upgrade. If you choose to cancel, the router remains in the unified ISSU initialized state. The CLI command set becomes fully accessible.

If you do not cancel at this point, then the process continues and any line modules that do not support unified ISSU are reloaded. Application sessions are brought down and traffic forwarding is interrupted for the unsupported modules.

If you do cancel in response to the CLI prompt, unified ISSU returns to the initialized state, and the following releases are present on the router:

- Primary SRP module—Running (original) release
- Redundant SRP module—Upgrade release; the module is in the unified ISSU initialized state
- Line modules—Running (original) release

To roll back from the unified ISSU initialized state, you must issue the issu stop command. The command reloads the redundant SRP module with the armed release and places unified ISSU in the idle state. As a result, the following releases are present on the router:

- Primary SRP module—Running (original) release
- Redundant SRP—Upgrade release
- Line modules—Running (original) release