Routers designed for high availability (HA) include hardware redundancy such as dual power supplies (PSU) and router processors (RP).
The RP is responsible for for learning the network topology and building the route table (RIB).
If the RP fails it can trigger routing protocol adjacencies to reset resulting in network instability.
During a RP failure it may be more desirable to hide the failure from the rest of the network and allow the router to continue forwarding packets using the previously programmed CEF table entries rather than drop packets and wait for the secondary router processor to rebuilt the forwarding table and re-establish adjacencies.
Stateful Switchover (SSO) is a redundancy feature that allows a Cisco device with two route processors to synchronise router configuration and control plane state information.
The processing of mirroring this information between route processors is known as checkpointing. Stateful Switchover enabled routers will always checkpoint line card operation and Layer 2 protocol states.
In a triggered failure event the standby route processor will immediately take control and prevent basic problems such as line interface flapping. Layer 3 forwarding is disrupted without additional configuration as the switchover will trigger a routing protocol adjacency flap that clears the route table.
When the route table is cleared, CEF entries will be purged and traffic no longer routed until the topology is relearned by the new route processor.
Enabling additional configuration in the form of nonstop forwarding (NSF) or nonstop routing (NSR) will keep CEF entries for a short duration in a failover event to keep packet forwarding in an event of an RP failure until the control plane has time to recover.