Issu Process Overview - Cisco Catalyst 4500 series Administration Manual

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 6-8 on page
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
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 6-3
Control
plane
Data
plane
Software Configuration Guide—Release IOS XE 3.6.0SG and IOS 15.2(2)SG
6-6
6-11.) Cisco IOS ISSU takes advantage of the Cisco IOS high availability
6-3).
High Availability Features and Hardware Redundancy in the ISSU Process
Active
Supervisor NSF/SSO
Engine
Line cards
Chapter 6 Configuring the Cisco IOS In-Service Software Upgrade Process
Management Management
Standby
plane plane
Supervisor
Engine
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