Cisco Catalyst 4500 Series Configuration Manual page 218

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 5-7
Figure 5-8
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
5-10
Load New Standby Supervisor Engine with New Cisco IOS Software
Standby is reset and
reloaded with
new software
Cisco IOS Cisco IOS
12.x(z)S 12.x(y)S
Standby
NSF/SSO
Supervisor
Engine
Line cards
shows the steps during the ISSU process.
Chapter 5 Configuring the Cisco IOS In-Service Software Upgrade Process
Cisco IOS Cisco IOS
12.x(z)S 12.x(y)S
Active
Supervisor
Engine
Figure 5-7).
OL-25340-01