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Network Warrior, 2nd Edition by Gary A. Donahue

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Disaster Chains

Modern commercial aircraft are some of the most amazingly redundant systems on Earth. When one device fails, another device takes over. When two systems fail, a third can take over with a reduced capacity, and so on. Commercial airplanes even have multiple pilots! Still, these testaments to fail-safe engineering can and do crash. When they do, the crash is usually discovered to have been caused by a series of events. These events, all relatively innocuous in and of themselves, spell disaster when strung together. Such a chain of events is called an accident chain.

Massively redundant networks can suffer from accident chains, too. Considering the impact such compound networking failures can have on a business (not to mention one’s paycheck), I like to use the term disaster chains to describe them.

Imagine a network with two Cisco 6509 switches in the core. An outage is planned to upgrade one, then the other. Because they’re in a redundant pair, one can be brought down without bringing down the network. The first switch is brought down without incident. But, as I’m working, I manage to get my foot tangled in the power cord of the other 6509, and pull it out of the power supply. Of course, the 6509 AC power supplies allow the power cables to be secured with clamps, but the last engineer to work on the switches forgot to retighten the clamps. Each 6509 has two power supplies, which are connected to different circuits, so pulling one power cord should not be an issue. However, ...

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