The term ‘High Availability’ is a very broad and general term that often means different things to different people and vendors. From a Cisco perspective, High Availability (HA) refers to the technology delivered in Cisco IOS Software that enables network-wide resilience to increase IP network availability. Network availability is a function of reliability and serviceability.
Availability is calculated mathematically and is often expressed in percentages, such as 99.999% availability. These percentages are calculated from failure rates, measured in Mean Time Before Failure (MTBF) (reliability), and repair times, measured in Mean Time To Repair (MTTR) (serviceability). In other words, both reliability and serviceability ultimately affect the availability of the network. Reliability and serviceability are described in the following sections.