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An in-service software upgrade (ISSU) is a process through which networking devices and firmware can be patched or upgraded without interrupting the underlying device/equipment’s operations. ISSU is a method for updating a networking device without the need to stop its ongoing processes and restart, which degrades overall network services.
ISSU is primarily used to ensure maximum network availability with minimal downtime as a result of device maintenance or upgrade processes. ISSU was initially introduced by Cisco Systems for updating and upgrading their suite of networking devices. For the ISSU to work, the networking/equipment must have a redundant route processor (RP) within its core architecture. This way, when updating, the upgrade process can be mapped onto a parallel RP and the routine operations/services can be performed from another RP. For example, some Cisco routers and switches are integrated with active and standby route processors. When the upgrade/patching process starts, the active RP's operations are switched to the standby RP until the upgrade process is complete.