In some scenarios, it is found that the Cisco IOS on Active and Standby Supervisor does not match hence it affects redundancy.
Catalyst 6500 Series Switches allow a redundant Supervisor Engine to take over if the primary Supervisor Engine fails in order to support fault resistance. The fast software upgrade (FSU) procedure supported by route processor redundancy (RPR) allows you to upgrade the Cisco IOS image on the Supervisor Engines without the need to reload the system.
Catalyst 4500 Series Switches allow a redundant Supervisor Engine to take over if the active Supervisor Engine fails. In software, Supervisor Engine redundancy is enabled if you run the redundant Supervisor Engine in RPR or Stateful Switchover (SSO) operating mode.