The Junos ISSU feature allows for a virtually hitless upgrade from one supported ISSU release to another. As mentioned previously, the foundation of ISSU is NSR, as during the ISSU process a GRES switchover occurs as the original standby becomes master to allow the old master (new backup) to be upgraded. Like GRES and NSR, ISSU can only be used on systems with redundant REs.
What is a supported ISSU release? In most cases, you are limited to no more than three major releases, and in theory any extended End-of-Life (EEOL) release to a current release should work. This section details current ISSU operation and restrictions for Trio-based MX routers running Junos 11.4.
At a high level, ISSU operation is pretty straightforward. Things
begin when you include the
in-service-upgrade switch rather than
add when using the
request system software command. The current
master then pushes the new software bundle to the BU RE, where it’s
installed. After the BU RE reboots with the new software, the current
master upgrades the PFE components in a sequential manner, which induces
a brief dataplane outage known as a dark window. Next, a GRES event is
triggered to make the old BU the new master, at which point the new BU
(old master) begins installing the new software. When all goes to plan,
you are left with both REs on the new software, now with RE1 as the
master, all with no control plane hit and only a brief period of
disruption to the data plane.