To perform an ISSU on a Junos device, what is a prerequisite related to the Routing Engine?

The prerequisite regarding the Routing Engine for performing an In-Service Software Upgrade (ISSU) on a Junos device is that graceful Routing Engine switchover must be enabled. This is crucial because ISSU allows for software upgrades on a live system with minimal disruption, and a graceful switchover ensures that the traffic is seamlessly redirected from one Routing Engine to another during the process.

By having this feature enabled, the system can switch over to the backup Routing Engine without dropping existing connections, which is vital for maintaining uptime and service continuity during the upgrade. This capability ensures that even with one Routing Engine undergoing the upgrade, the other continues to process traffic smoothly.