Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.0E and IOS 15.2(5)E
Chapter 7 Configuring the Cisco IOS In-Service Software Upgrade Process
About ISSU
ISSU Process Overview
The ISSU process allows you to perform a Cisco IOS 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 7-8 on page 7-11 .) Cisco IOS ISSU takes advantage of the Cisco IOS high availability
infrastructure—Cisco NSF with SSO and hardware redundancy—and eliminates downtime associated
with software upgrades or version changes by allowing changes while the system remains in service (see
Figure 7-3 ).
SSO and NSF mode support configuration and runtime state synchronization from the active to the
standby supervisor engine. For this process to happen, the images 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 support different sets of features and commands.
Figure 7-3 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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