For routers with redundant REs, unified ISSU allows the administrator to upgrade between two different JUNOS software releases with no disruption on the control plane and with minimal disruption of traffic.
Although the goal of this book is to discuss practical application
of tools and technologies rather than protocol theory, let’s take a
quick look at how unified ISSU performs its magic. The process begins
command is executed on the master RE:
run request system software in-service-upgrade
After this command is entered, the router begins an eight-step sequence to complete the upgrade:
The router validates the configuration to make sure all elements are compatible with the new revision of code, in this case 9.3R1.7. The router also checks to make sure that all Physical Interface Cards (PICs) are compatible with ISSU. If an incompatible PIC is found, the upgrade continues but the router issues a warning.
Kernel state on the backup RE is synchronized with the master RE.
The configuration file on the master RE is copied to the backup RE.
The backup RE is upgraded with the new operating system, and then kernel state is resynchronized to the master.
chassisd on the master RE
notifies the Packet Forwarding Engine (PFE) on each Flexible PIC Concentrator (FPC) to download the new
image from the backup RE.
The PFEs download the image, then reboot. Following ...