What are three necessary actions for performing a unified in-service software upgrade (ISSU) on a Junos device?

For performing a unified in-service software upgrade (ISSU) on a Junos device, enabling graceful Routing Engine switchover (GRES) is a necessary action because it allows for continuous forwarding of traffic while the upgrade process is taking place. GRES provides redundancy by allowing one Routing Engine (RE) to take over the functions of another without losing session information or data traffic. This ensures that the device maintains availability and stability during the upgrade, which is a crucial aspect of in-service upgrades.

When GRES is enabled, if a failure occurs or if the software needs an upgrade, the functioning RE continues to handle data traffic while the other RE is taken offline momentarily for the upgrade. This capability is essential in environments where uptime is critical, minimizing service disruption.

The other actions listed may have their own significance in different contexts but are not strictly necessary for the process of performing an ISSU. For instance, ensuring that user sessions are logged out might be important in specific maintenance scenarios, but the ISSU process specifically relies on the ability of GRES to handle existing sessions gracefully.