Unified In-Service Software Upgrade (ISSU)

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Unified ISSU enables you to upgrade between two different Junos OS releases with no disruption on the control plane and with minimal disruption of traffic.

Unified ISSUE is supported only on dual RE platforms. In addition, GRES and NSR must be enabled. The RE's must be running the same software release before you can perform a unified ISSU.

Unified In-Service Software Upgrade (ISSU) is a feature that enables network operators to upgrade or downgrade the Junos OS software version on a device without disrupting network services. Here are the two correct statements about Unified ISSU:

A. Unified ISSU is supported only on dual Routing Engine (RE) platforms

This statement is true. Unified ISSU is a feature that is only supported on platforms with dual Routing Engines. Dual Routing Engine platforms provide redundancy and resilience in case of hardware or software failures. When performing an ISSU, the system updates one RE at a time, allowing the other RE to continue to forward traffic and maintain network services during the upgrade.

B. To perform a unified ISSU, it is not necessary for the master RE and backup RE to be running the same version of the Junos OS

This statement is also true. Unified ISSU allows network operators to upgrade or downgrade the Junos OS version on a device without disrupting network services. To achieve this, the ISSU process loads the new version of the Junos OS on the standby RE first, then synchronizes the configuration and state information between the active and standby REs. Once the synchronization is complete, the system performs a switchover to the standby RE, which becomes the new active RE. The old active RE is then upgraded to the new version of Junos OS, and the process repeats for the standby RE.

C. Unified ISSUE causes minimal disruption on both the control and forwarding planes

This statement is true. Unified ISSU is designed to minimize service disruption during the upgrade process. By updating the Junos OS software on one RE at a time, the system ensures that network services continue to run without interruption. Unified ISSU also provides protection against service interruption in case of hardware or software failures during the upgrade process.

D. Graceful Routing Engine Switchover (GRES) and nonstop active routing (NSR) must be enabled to perform a unified ISSU

This statement is false. Although Graceful Routing Engine Switchover (GRES) and nonstop active routing (NSR) are important features for high availability and resiliency, they are not required for Unified ISSU. Unified ISSU is designed to minimize service disruption during software upgrades or downgrades, and it works on its own without relying on any other specific features. However, enabling GRES and NSR can provide additional protection against service interruption in case of hardware or software failures during the upgrade process.