In information technology, high availability refers to a system or component that is continuously operational for a desirably long length of time. Availability can be measured relative to “100% operational” or “never failing.” A widely-held but difficult-to-achieve standard of availability for a system or product is known as “five 9s” (99.999 percent) availability.
Since a computer system or a network consists of many parts in which all parts usually need to be present in order for the whole to be operational, much planning for high availability centers around backup and failover processing and data storage and access. For storage, a redundant array of independent disks (RAID) is one approach. A more recent approach is the storage area network (SAN).
Some availability experts emphasize that, for any system to be highly available, the parts of a system should be well-designed and thoroughly tested before they are used. For example, a new application program that has not been thoroughly tested is likely to become a frequent point-of-breakdown in a production system.
Technology used when creating HA networks are for example:
- HSRP – Hot Standby Router Protocol is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway, and has been described in detail in RFC 2281.
- VRRP – The Virtual Router Redundancy Protocol (VRRP) is a computer networking protocol that provides for automatic assignment of available Internet Protocol (IP) routers to participating hosts. This increases the availability and reliability of routing paths via automatic default gateway selections on an IP sub network. Described in detail in RFC 5798.
- Multicast mac