Have you ever been in a traffic jam? Ever been in gridlock? Ever had to take a detour rather than the normal road to your destination?

Computer networks face similar traffic problems. A backhoe tractor might be digging ground for a new pipe and accidentally sever a network cable buried underground. A person might accidentally misconfigure a router, bringing the network crashing down. Or a building may lose power.

Network designers must anticipate these possibilities and design their networks accordingly. Redundant network paths (such as a road detour) and redundant devices ensure that if one device fails or one path becomes unavailable, you can select another with minimal delay. Protocols facilitate the quick recovery of a failure so that network devices can relearn the new paths to use.

Network redundancy is costly, so network designers must weigh the increased cost of implementing network redundancy and disaster recovery against the risk of not being able to recover from a failure. In this part, you’ll learn about the methods, benefits, and costs of keeping the network running.