ABSTRACT
Most routers with advanced architectures will be deployed in the core of corporate networks and in the core of the Internet and the Internet2; a smaller portion of them will be deployed in end customer locations that are not the core or mission-critical parts of the network. While most of the routers in end customer locations will not have to fulfill High Availability (HA) criteria, those in the core of the network-be it a corporate network or the Internet-will. HA is typically defined as fully functional operational status of a device or a network in excess of 99.999%. This is why it is sometimes referred to as “five nines.” In other words, the availability of a device or a network is defined in terms of a probability to function as intended, and thus to be available to perform its intended function. Since every device or network consists of a large number of individual components, each of which have their very own life span (and therefore a certain failure probability), the fully functional operational status is possible only when all necessary components function as intended-and that includes their software. As a result, the failure probability of a complex device such as a router is significantly higher than the failure probability of each of its components if no provisions are taken to increase the reliability of the system. As with all compound systems, the failure probability is the compound failure probability of each of the components or subsystems.
