Cisco Nexus – Upgrading with In-Service Software Upgrade (ISSU)

In the fast-paced world of networking, system uptime is a top priority. When upgrading network infrastructure, minimizing downtime is essential. Cisco Nexus 9000 Series switches offer an In-Service Software Upgrade (ISSU) feature, which greatly reduces or eliminates downtime during the software upgrade process.

What is ISSU?

ISSU is a powerful feature of the Cisco Nexus 9000 Series NX-OS, designed to perform software upgrades without disrupting system operations. It leverages existing features like Nonstop Forwarding (NSF) and Stateful Switchover (SSO) to ensure the continuity of data plane traffic. Control plane downtime is reduced to approximately less than 6 seconds.

In redundant systems with dual supervisors, one supervisor is active while the other operates in standby mode. ISSU loads the new software onto the standby supervisor while the active supervisor continues to operate with the old software. During the upgrade, a switchover occurs – the standby supervisor becomes active, running the new software. Subsequently, the new software is loaded onto the now standby supervisor.

Noteworthy Points

  • ISSU is not supported on Nexus 9504, 9508, or 9516 chassis with N9K-C95xx-FM-Ex or N9K-C950x-FM-R fabric modules. Upgrades in these scenarios will be disruptive.
  • During the upgrade, configuration settings or network connections must not be changed. Alterations may cause a disruptive upgrade.
  • Configuration mode is blocked during the ISSU process to prevent changes.
  • Downgrades are only supported in a disruptive manner. Non-disruptive downgrades are not supported.

How ISSU Works

The ISSU process on the Cisco Nexus 9000 Series chassis with dual supervisors is as follows:

1. The process starts when the administrator executes the `install all` command.

2. It verifies the location and integrity of the new software image file.

3. It checks the operational status, supervisors’ current software versions, and all switching modules to ensure ISSU capability.

4. The new software image is loaded onto the standby supervisor and brought to the HA-ready state.

5. A supervisor switchover is forced.

6. The new software image is loaded onto the now standby supervisor, bringing it to the HA-ready state.

7. A non-disruptive upgrade of each switching module is performed.

Throughout the upgrade process, detailed status information is presented on the console, and administrator confirmation is requested at crucial steps.

Determining ISSU Compatibility

Before initiating the ISSU, it is crucial to ensure compatibility. Some configurations or features on your current software might not be supported on the new image, rendering ISSU disruptive. Use the following command to determine ISSU compatibility:

show install all impact nxos bootflash:nxos-image.bin

This command verifies whether the features configured are compatible with the new NX-OS image. It also evaluates the impact of the upgrade on the entire system. It checks for prerequisites, verifies the integrity of the new image, and assesses whether the switch hardware is compatible with the new image. It provides detailed information about what will happen during the upgrade process, including which components will be restarted and whether or not the upgrade will be disruptive (i.e., if a reload is necessary).

Example ISSU Process

Below is an example CLI sequence for performing an ISSU on Cisco Nexus 9000 Series switches:

Step 1: Copy the new image to the switch
copy scp://username@server/path/nxos-image.bin bootflash:

Step 2: Verify ISSU compatibility
show install all impact nxos bootflash:nxos-image.bin

Step 3: Initiate ISSU
install all nxos bootflash:nxos-image.bin

Step 4: Monitor and confirm steps as prompted by the system
The system will prompt for confirmations and display status information.

Step 5: Verify the upgrade
show version


ISSU is a powerful feature in Cisco Nexus 9000 Series switches that allow for software upgrades with minimal disruption. Careful planning and following best practices ensure the upgrade process is smooth and efficient. Always consult Cisco documentation for the latest information and guidelines.