Network Sales and Services Handbook (Cisco Press Networking Technology)

High availability is a subjective interpretation of two statistics:

• MTTR Average time required to recover/restore from a network outage or repair a failed component.

• MTBF Average time a network or component works without failure.

Network availability is measured by taking the uptime and dividing it by the uptime plus the downtime, then multiplying the result by 100, resulting in a percentage:

Availability = ((Uptime)/(Uptime + Downtime) x 100

Network designers use the required network availability to determine how much protection and redundancy to build into the network.

The level of application and network availability is balanced against the cost of implementing the redundant/backup systems. It is not uncommon for the cost of making a network completely redundant or fault tolerant to be prohibitive.

There are various approaches available to a network administrator/designer in creating network redundancy. These approaches are as follows:

• Redundant WAN Links Multiple WAN access links providing local loop access redundancy.

• Meshed Network Topologies Enables interconnectivity of every node on a network with every other node.

• Redundant Power Systems Diverse power feeds or an extended-battery UPS.

• Fault-Tolerant Media Dual-NICs preventing network media outages from taking down a network segment.

• Hardware Backup Dual networking hardware devices, such as routers, switches, hubs, and bridges.