An ISSU (in-service software upgrade) is a technique for updating software on a network device without taking that device offline and thereby disrupting network services.
In-service software upgrades allow a network operator to address software bugs and add new features to switches and routers without interrupting network availability. An ISSU eliminates the need to reboot the entire device. Vendors vary in their approaches to ISSU and not all of their products support the feature. Most vendors offer ISSU on their premium routers and switches, particularly core routers and switches.
Generally an ISSU requires a network device with redundant control plane elements, such as supervisor engines or routing engines. This redundancy allows a network operator to update the software image on one engine while the other maintains network availability. Depending on a particular vendor’s architecture, other elements on the switch or router will also have to reboot, which will result some degradation of service. The network can adjust to this with minimal disruption if the network operator configures and executes the ISSU properly.