Companies are increasingly looking for 24 hour, seven day a week uptime for their computer networks. Achieving 100% uptime is perhaps impossible but securing a 99.999% or five nines uptime is a goal that organizations set. This is interpreted to mean one day of downtime, on average, for every 30 years, or one hour of downtime, on average, for every 4000 days, or 5.25 minutes of downtime per year.

Achieving such a goal requires extremely reliable networks. Reliability in networks is achieved by reliable equipment and by designing networks that are tolerant to failures and faults. The network is designed to reconverge rapidly so that the fault is bypassed.

Fault tolerance is achieved by redundancy. Redundancy means to be in excess or exceeding what is usual and natural. How does redundancy help achieve reliability?

Assume that the only way to get to work is by a car. If the car develops a fault that makes it unusable, going to work will be impossible until it is repaired and returned.

If the car fails and is unavailable, on average one day in ten then there is 90% usage. Going to work is possible nine days in every ten. Reliability is therefore 90%.

Buying another car will improve matters. There is no need for two cars just to get to work. However, it does provide redundancy (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. Purchasing a second redundant car has improved reliability to 99%.

Web Links Redundancy and Broadcast Storms http://www.howstuffworks.com/ lan-switch5.htm