Implementing Hardware and Software Redundancy on Modular Switches
What is stateless switchover – SSO?

When a redundant supervisor engine runs in SSO mode, the redundant supervisor engine starts up in a fully-initialized state and synchronizes with the persistent configuration and the running configuration of the active supervisor engine. It subsequently maintains the state of the Layer 2 protocols, and all changes in hardware and software states for features that support stateful switchover are kept in sync. Consequently, it offers zero interruption to Layer 2 sessions in a redundant supervisor engine configuration. SSO is supported in 12.2(20)EWA and later releases.

Because the redundant supervisor engine recognizes the hardware link status of every link, ports that were active before the switchover will remain active, including the uplink ports. However, because uplink ports are physically on the supervisor engine, they will be disconnected ONLY if the supervisor engine is removed.

If the active supervisor engine fails, the redundant supervisor engine becomes active. This newly active supervisor engine uses existing Layer 2 switching information to continue forwarding traffic. Layer 3 forwarding will be delayed until the routing tables have been repopulated in the newly active supervisor engine.