Upgrading Using Unified ISSU
Unified in-service software upgrade (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 in-service software upgrade is only supported by dual Routing Engine platforms. In addition, graceful Routing Engine switchover (GRES) and nonstop active routing (NSR) must be enabled. For additional information about using unified in-service software upgrade, see the High Availability Feature Guide for Routing Devices.
For information about unified ISSU support across platforms and Junos OS releases, see the In-Service Software Upgrade (ISSU) Web application.