Cisco Catalyst 4500 Series Software Configuration Manual page 409

Chapter 8 Configuring the Cisco IOS XE In Service Software Upgrade Process
The former active supervisor engine is loaded with an old Cisco IOS XE 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 software image. Next, the former active supervisor engine is loaded
with the new version of Cisco IOS XE software and becomes the new standby supervisor engine (see
Figure
Figure 8-7
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
8-7).
Load New Standby Supervisor Engine with New Cisco IOS XE Software
Standby is reset and
reloaded with
new software
Cisco IOS XE Cisco IOS XE Cisco IOS XE
3.z.y.SG 3.z.y.SG 3.z.y.SG
Standby
NSF/SSO
Supervisor
Engine
Line cards
Cisco IOS XE Cisco IOS XE Cisco IOS XE
3.z.y.SG 3.z.y.SG 3.z.y.SG
Active
Supervisor
Engine
About Performing ISSU
8-11