What feature must be enabled prior to performing a unified in-service software upgrade?

Enabling Graceful Routing Engine Switchover (GRES) is crucial before performing a unified in-service software upgrade. GRES allows for the seamless switchover of the Routing Engine from one instance to another without interrupting the flow of traffic. This is essential during a software upgrade because it ensures that the network remains operational, minimizing downtime and maintaining session continuity for active connections.

When GRES is enabled, the system can intelligently manage the routing information and critical services while the upgrade process occurs, which helps in sustaining high availability. This is particularly important in environments where constant uptime is necessary, such as in enterprise or service provider networks.

Other options, while they may relate to network management and performance, do not specifically facilitate the smooth upgrade of software within a unified in-service context. Thus, GRES stands out as the key requirement to ensure that such upgrades can be carried out effectively without disrupting ongoing services.