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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
The former active supervisor engine is loaded with an old Cisco IOS image so that if the new active
supervisor engine experiences problems, you can abort and conduct a switchover to the former active,
which is already running the old image. Next, the former active supervisor engine is loaded with the new
version of Cisco IOS software and becomes the new standby supervisor engine (see Figure 7-7).
Figure 7-7 Load New Standby Supervisor Engine with New Cisco IOS Software
Figure 7-8 shows the steps during the ISSU process.
Line cards
Standby
Supervisor
Engine
Cisco IOS
12.x(y)S
Cisco IOS
12.x(y)S
Cisco IOS
12.x(z)S
Standby is reset and
reloaded with
new software
Cisco IOS
12.x(z)S
Active
Supervisor
Engine
180234
NSF/SSO
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