Cisco Catalyst 4500 series Administration Manual page 272

In-Service Software Upgrade (ISSU) on a VSS
Figure 5-9
Note that at any given instant, at least one of the switches is Active. The Active
switch is in operation, i.e. continues to forward traffic and participate in network
control protocols throughout the duration of the upgrade operation.
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)SG
5-56
Upgrading VSS System
Active
(version X)
VSL
Active
(version X)
VSL
Active
(version X)
VSL
Statefull Switchover
(Reboot)
VSL
Standby
(version X)
VSL
(Reboot)
VSL
Standby
(version Y)
VSL
Chapter 5 Configuring Virtual Switching Systems
Standby
(version X)
(Reboot)
Standby
(version Y)
Active
(version Y)
Active
(version Y)
Active
(version Y)
Active
(version Y)
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