Prior to the implementation of the Broadband High Availability In-Service Software Upgrade feature, software upgrades typically
required planned outages that took the router or network out of service. The Broadband High Availability In-Service Software
Upgrade feature enables the service provider to maximize network availability and eliminate planned outages by allowing the
Cisco IOS XE release to be upgraded without taking the router or network out of service. ISSU is a procedure, based on Cisco
high availability (HA) architecture, whereby the Cisco IOS XE infrastructure accomplishes an upgrade while packet forwarding
continues and broadband sessions are maintained. Cisco HA architecture is based on redundant Route Processors and the NSF
and SSO features, such that ports stay active and calls do not drop, eliminating network disruption during upgrades.
The ISSU feature allows deployment of new features, hardware, services, and maintenance fixes in a procedure that is seamless
to end users. A critical component of ISSU and Cisco HA technology is the cluster control manager (CCM) that manages session
recreation and synchronization on the standby processor. The Broadband High Availability In-Service Software Upgrade feature
allows the configuration of subscriber redundancy policies that tune the synchronization process. For more information see
the Configuring Subscriber Redundancy Policy for Broadband High Availability In-Service Software Upgrade.
The Broadband High Availability In-Service Software Upgrade feature handles upgrades and downgrades, and supports the following:
Upgrades from one software feature release to another, as long as both versions support the ISSU feature, for example, from
Cisco IOS XE Release 2.2 to Cisco IOS XE Release 2.3.
Upgrades from one software maintenance release to another, for example from Cisco IOS XE Release 2.2.1 to Cisco IOS XE Release
The Broadband High Availability In-Service Software Upgrade feature works with other Cisco IOS XE HA features, NSF and SSO,
to maintain broadband sessions.