Information About HSRP - ISSU
The In Service Software Upgrade (ISSU) process allows Cisco software to be updated or otherwise modified while packet forwarding continues. In most networks, planned software upgrades are a significant cause of downtime. ISSU allows Cisco software to be modified while packet forwarding continues, which increases network availability and reduces downtime caused by planned software upgrades.
For detailed information about ISSU, see the Cisco IOS In Service Software Upgrade Process document in the High Availability Configuration Guide.