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Redundant Topologies / Redundancy

Redundancy
7.1.1 This page will explain how redundancy can improve network reliability and performance.
Many companies and organizations increasingly rely on computer networks for their operations. Access to file servers, databases, the Internet, intranets, and extranets is critical for successful businesses. If the network is down, productivity and customer satisfaction decline.
Increasingly, companies require continuous network availability, or uptime. 100 percent uptime is perhaps impossible, but many organizations try to achieve 99.999 percent, or five nines, uptime. Extremely reliable networks are required to achieve this goal. This is interpreted to mean one hour of downtime, on average, for every 4,000 days, or approximately 5.25 minutes of downtime per year. To achieve such a goal requires extremely reliable networks.
Network reliability is achieved through reliable equipment and network designs that are tolerant to failures and faults. Networks should be designed to reconverge rapidly so that the fault is bypassed.
Figure illustrates redundancy. Assume that a car must be used to get to work. If the car has a fault that makes it unusable, it is impossible to use the car to go to work until it is repaired.
On average, if the car is unuseable due to failure one day out of ten, the car has ninety percent usage. Therefore, reliability is also 90 percent.
A second car will improve matters. There is no need for two cars just to get to work. However, it does provide redundancy, or backup, in case the primary vehicle fails. The ability to get to work is no longer dependent on a single car.
Both cars may become unusable simultaneously, one day in every 100. The second car raises reliability to 99 percent. 
The next page will discuss redundant topologies

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