Issu Overview; Issu Methods - HP 12500 Configuration Manual

ISSU overview

The In-Service Software Upgrade (ISSU) function enables a fast software upgrade without (or with the
least) service interruption. During an ISSU, you can perform version rollback and use display commands
to view the version compatibility and upgrade status.
ISSU is implemented on the basis of the following design advantages:
Separation of service features from basic functions. The software of the device includes a boot
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image, a system image, some feature images, and some patch images (if any). The boot image and
system image provide the operating system. Feature images provide service features. Usually, you
only need to add new service features or upgrade existing service features, without affecting the
basic system functions.
Independence between service features. When you add or upgrade one service feature, the other
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service features and the system operation are not affected.
Support for hotfix. By installing patch images, you can fix system bugs without rebooting the device.
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Hardware redundancy. When the device has two MPUs and is operating in standalone mode, the
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standby MPU can take over while the active MPU needs to reboot for an upgrade. When the device
is operating in IRF mode, a standby MPU of the IRF fabric can take over while the global active MPU
needs to reboot for an upgrade.
• For more information about images, see "Upgrading software."

ISSU methods

The device provides several ISSU methods for upgrades to compatible versions. When you perform an
ISSU, the device compares the compatibility between the old and new software versions and selects an
ISSU method based on the version compatibility.
Before an ISSU, you can check for the ISSU method to be used for the upgrade and view the possible
impact of the upgrade. Whichever ISSU method is used, the ISSU procedure and commands are the
same.
The device provides the following categories of ISSU methods:
Incremental upgrade
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An incremental upgrade analyzes the differences between the new and old software versions and
upgrades only the different parts. An incremental upgrade takes the least time and imposes the
least effect on the device.
Incremental upgrade methods include:
Service-level incremental upgrade—This method involves only the upgraded service modules.
The other service modules provide services normally during the upgrade.
File-level incremental upgrade—This method involves only hidden system program files. The
system operates and provides services normally during the upgrade.
ISSU reboot
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An ISSU reboot upgrade saves the current system information (including the operation data,
configuration data, hardware data, and status information) to the memory and uses the new
software to reboot the CPU. During an ISSU reboot upgrade, the data forwarding plane of the
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