Software outages are challenging even when baseline hardware redundancy is in place. There should be a balance to provide high availability routing otherwise router problems typically propagate not only throughout the service provider network, but also externally to other connected networks possibly belonging to other service providers. This could affect customers on a broad scale. Presently, there are several software availability features that contribute to the percentage of time that a router is available to process and forward traffic.
To fully appreciate high availability, you should realize that all routing protocols specify minimum time intervals in which the peer device must receive an acknowledgment before it disconnects the session.
OSPF default session timeout is approximately 40 seconds. The timeout intervals are configurable.
BGP default session timeout is approximately 120 seconds. The timeout intervals are configurable for the 7750 SR and 7950 XRS only.
Therefore, router software has to recover faster than the specified time interval to maintain up time.