Cisco Stacks – Network Upgrades with ISSU and Cisco StackWise Virtual

In today’s fast-paced world, network reliability and uptime are critical for any business. However, network administrators often face a daunting task when upgrading the software of network devices, as traditionally, this process has been associated with network downtime. This is where Cisco’s In-Service Software Upgrade (ISSU) comes to the rescue. ISSU allows network administrators to upgrade the software of network devices with minimal or no network disruption. This post will delve into what ISSU is, how it works, and how it integrates with Cisco StackWise Virtual for an enhanced upgrade experience.

What is ISSU?

ISSU stands for In-Service Software Upgrade. It is a process that allows the upgrade of the software image on a network device while the device continues to forward packets. This means that the network doesn’t experience any significant downtime during the upgrade. ISSU is especially helpful for network administrators as it allows them to perform software upgrades without the fear of causing a network outage. The key aspect of ISSU is that it upgrades the images in install mode, where each package is upgraded individually.

ISSU is versatile, as it supports software upgrades and rollback. This can be performed in single or three steps, offering flexibility based on the specific use case and requirements.

Cisco StackWise Virtual and ISSU

Cisco StackWise Virtual is a network system that combines two switches into a single logical switch. This is achieved by connecting two switches using high-bandwidth connections. StackWise Virtual enhances ISSU by allowing one of the switches to take over the network traffic while the other is being upgraded.

For detailed configuration information on Cisco StackWise Virtual, refer to the “Configuring Cisco StackWise Virtual” chapter in the Cisco guide.

ISSU Upgrade Process

Let’s break down the steps involved in performing an ISSU:

1. Copy the New Image: The first step involves copying the new software image to both the standby and active switches in the StackWise Virtual pair.

2. Unzip and Copy Packages: Next, unzip the files and copy the individual packages to both the active and standby switches.

3. Install Packages on Standby Switch: You need to install the software packages on the standby switch. This switch is not actively forwarding traffic, so it can be upgraded without affecting the network.

4. Restart the Standby Switch: Once the installation is complete, restart the standby switch. At this point, the standby switch will be running the new software.

5. Install Packages on the Active Switch: It’s time to install the software packages on the active switch.

6. Restart the Active Switch and Perform Switchover: After installing the packages on the active switch, it needs to be restarted. During this restart, the standby switch (which has already been upgraded) takes over as the new active switch. Once the former active switch returns online, it becomes the new standby switch.

7. ISSU Completion: At this stage, both switches have been successfully upgraded. The new software image is installed on both switches, and ISSU is completed.

Important Consideration: Impact on Connected Devices

It’s crucial to understand that while ISSU minimizes the network downtime, devices directly connected to the switches will experience downtime when the switches are rebooted. There’s no way around this because the physical switch must reboot to complete the software upgrade, causing temporary loss of connectivity for devices plugged into it. This can be impactful, especially for sensitive applications and services. Therefore, planning the upgrade carefully, ideally during maintenance windows, and communicating with stakeholders about the expected temporary disruption is essential.


ISSU is a powerful tool for network administrators, enabling them to perform software upgrades on network infrastructure with minimized network disruption. However, it’s essential to recognize that connected devices will experience downtime due to necessary switch reboots. Integrating ISSU with Cisco StackWise Virtual makes the process more efficient, as two switches work together to ensure continuous packet forwarding during the upgrade. This combination of technologies helps maintain high network availability and reliability, which is critical for modern businesses. Network engineers must plan and communicate effectively to mitigate the impact on connected devices during the upgrade process.

Example Configuration

Step 1: Copy the new image to the switch
copy flash:

Step 2: Unzip and copy packages
request platform software package expand switch all file flash:new-image.tar

Step 3: Install the packages on the standby switch (optional)
# This step might not be needed as the next steps take care of this in some scenarios

Step 4: Restart the standby switch (optional)
# This step might not be needed as the next steps take care of this in some scenarios

Step 5 and 6: Install the packages on the active switch and perform switchover
request platform software package install switch all file flash:new-image.tar auto-copy

Step 7: Verify the software version
show version