High Availability
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Support for node-by-node upgrade by disabling GRES (PTX10004, PTX10008, and PTX10016)—By default, Junos OS Evolved upgrades the entire cluster during a maintenance window, including both Routing Engines and all Flexible PIC Concentrators (FPCs). You can now upgrade the device one Routing Engine at a time by temporarily disabling GRES, upgrading the hardware backup Routing Engine first, switching over to it, then upgrading the new backup Routing Engine and re-enabling GRES.
[See Understand Graceful Routing Engine Switchover for Junos OS Evolved.]