In the modern, data-driven world it’s not uncommon for IT infrastructure to be running both hardware and software that is several generations behind the current technology.
Competing priorities can make getting downtimes approved difficult and often nearly impossible for hardware that isn’t actively showing a problem. Nonetheless preventative upgrades are an important part of the extending the lifecycle of the infrastructure, which is where the benefits of in-service software upgrades (ISSU) really shine.
The monolithic hardware systems from the past used to require full reboots and cold stops to upgrade software, but those days are behind us now. Even in lower cost, single supervisor switches we can achieve software upgrades that leave the users unaffected.
The Cisco Nexus top-of-rack platform is an excellent example of this technology. The Cisco Nexus 9300s only have a single supervisor, but during an ISSU upgrade the Nexus allows the data plane to run unmanaged briefly as the supervisor CPU resets loads the target upgrade image. Once the control plane is updated and booted successfully it syncs with the runtime state pre-upgrade and reconciles that with the data plane. This allows traffic to continue to pass, unaffected by the in-progress system upgrade.
Of course, before beginning with any sort of ISSU procedure compatibility should be checked to ensure that there is no chance of disruption to the services provided.
The first step should be checking the release notes for the target release. This will have a list of ISSU supported releases. If you’re current version is not listed you may need to stair-step the version up, so pick one of the ones off the list, check its release notes and work backwards.
Once you’ve got your firmware picked and you’ve loaded it onto the switch you’ll also want to check and make sure there are no incompatible features and that all modules will be non-disruptive. This can be achieved by running the show incompatibility system bootflash: