I've been hearing about ISSU for a while now, but never really knew what it was. Finally, when ACI started supporting it I thought maybe I should look into it. ISSU stands for In-Service Software Upgrade, which is used in different ways by different vendors. But the gist is that it allows admins to upgrade their switches and routers without losing network availability.
Though different vendors handle it differently, it seems a lot like Linux to me. You can upgrade a certain part of the software and restart that service, rather than restarting the whole device. Granted you should probably still utilize some redundancy or maintenance window.