Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 8 Configuring the Cisco IOS XE In Service Software Upgrade Process
About Performing ISSU
ISSU Process
The ISSU process allows you to perform a Cisco IOS XE software upgrade or downgrade while the
system continues to forward packets. (For an illustration of the commands used during the ISSU process,
refer to
Figure 8-8 .) Cisco IOS XE ISSU takes advantage of the Cisco IOS XE high availability
infrastructure—Cisco NSF with SSO and hardware redundancy—and eliminates downtime associated
with software upgrades by allowing changes while the system remains in service (see
Figure 8-3 ).
SSO and NSF mode support configuration and runtime state synchronization from the active to the
standby supervisor engine. For this process, the IOS XE software image on both the active and the
standby supervisor engines must be the same. When images on active and standby supervisor engines
are different, ISSU allows the two supervisor engines to be kept in synchronization even when these two
versions of Cisco IOS XE support different sets of features and commands.
Figure 8-3 High Availability Features and Hardware Redundancy in the ISSU Process
Management
plane
Management
plane
Line cards
Control
plane
Data
plane
Active
Supervisor
Engine
Standby
Supervisor
Engine
NSF/SSO
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