Cisco Cisco IOS Software Release 12.0(22)S

This document describes the Stateful Switchover (SSO) feature in Cisco IOS Release 12.0(22)S. It 
includes the following sections:

Development of the SSO feature is an incremental step within an overall program to improve the 
availability of networks constructed with Cisco IOS routers. 

is particularly useful at the network edgeSSO provides protection for network edge devices with dual 
Route Processors (RPs) that represent a single point of failure in the network design, and where an 
outage might result in loss of service for customers.

In specific Cisco networking devices that support dual RPs, SSO takes advantage of RP redundancy to 
increase network availability. The SSO feature takes advantage of RP redundancy by establishing one of 
the RPs as the active processor while the other RP is designated as the standby processor, and then 
synchronizing critical state information between them. Following an initial synchronization between the 
two processors, SSO dynamically maintains RP state information between them.

A switchover from the active to the standby processor occurs when the active RP fails, is removed from 
the networking device, or is manually taken down for maintenance. 

12.0(22)S

This feature was introduced.