High Availability

If a network switch or router stops operating correctly (meaning that a network fault occurs), communication through the network could be disrupted, resulting in a network becoming unavailable to its users. Therefore, network availability, called uptime, is a major design consideration. This consideration might, for example, lead you to add fault-tolerant devices and fault-tolerant links between those devices. This section discusses the measurement of high availability along with a collection of high-availability design considerations.

High-Availability Measurement

The availability of a network is measured by its uptime during a year. For example, if a network has five nines of availability, it is up 99.999 percent of the time, which translates to a maximum of 5 minutes of downtime per year. If a network has six nines of availability (it’s up 99.9999 percent of the time), it is down less than 30 seconds per year.