ISSU
What is ISSU?
ISSU (In-Service Software Upgrade) enables online software upgrades without service interruption. ISSU upgrades the device's OS or software version while keeping services uninterrupted. It is commonly used for software version updates on network devices such as routers, switches, and optical transmission equipment. ISSU is crucial for high availability and continuous service requirements, ensuring seamless upgrades while maintaining service continuity, reducing risks, and providing flexibility.
How Does ISSU Work?
ISSU upgrades the active and standby main processing units (MPUs) separately. It ensures data synchronization between different software versions. It also utilizes key technologies such as forwarding and control separation to minimize service interruption during the upgrade process.
The ISSU process includes the following stages:
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Preparation: Verify that the device meets ISSU upgrade requirements and that dual systems are available. The primary system remains active, while the standby MPU and upgrades to the new software version.
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Start: The backup system synchronizes with the primary system's operational state, and data backup occurs between the active and standby MPUS.
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Switchover: The standby MPU becomes the new active MPU, and Line Processing Units (LPUs) restart.
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End: The original active MPU restarts with the new system software, and after rebooting, it functions as the new standby MPU. The ISSU process is complete.
Key benefits of ISSU include the following:
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High Availability: Services remain uninterrupted during the upgrade, enhancing system reliability.
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Seamless Experience: Users remain unaware of the upgrade, ensuring an uninterrupted experience.
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Operational Efficiency: Minimizes downtime, reducing maintenance costs.
Traditional Upgrade vs. ISSU
ISSU enhances the upgrade efficiency of network devices through a non-disruptive, automated, and rollback-capable approach, reducing the risk of service interruptions caused by upgrades. It is particularly suited for industries with high availability requirements, such as finance, telecommunications, and cloud computing. The diagram below illustrates the differences between traditional upgrades (e.g., reboot-based upgrades) and ISSU.
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Features
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Traditional Upgrade
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ISSU
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Control Surface
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The control plane is coupled with the forwarding plane.
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The control plane is decoupled from the forwarding plane.
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Minimal Downtime
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The device needs to be restarted, causing service interruptions and impacting network availability.
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By using dual control planes, data plane forwarding is maintained uninterrupted during the upgrade process, enabling seamless upgrades.
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Business Continuity
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The traffic forwarding service is interrupted for minutes.
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The traffic forwarding service is interrupted for seconds.
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Automated Process
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The upgrade process needs to be performed manually, which carries the risk of complexity and potential errors.
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The upgrade process is automated, reducing the operational burden on maintenance personnel and minimizing the potential for human error.
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Rollback Capability
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If issues arise with the new version, a manual downgrade may be required, potentially leading to prolonged service recovery.
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A version rollback mechanism is provided, allowing for quick restoration to the previous stable version, enhancing network controllability and security.
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