Issues and Strategies of Redundancy

Keep the following two goals in mind when adding redundancy to a hierarchical design:

• Redundant paths should be used only when the normal path is broken, unless the paths are carefully engineered for load balancing. Although a network can use redundant links for load sharing as well as redundancy, this should be the exception rather than the rule. Load sharing must be carefully engineered to anticipate and prevent network instability when failures occur.

• Traffic shouldn't pass through devices or links that aren't designed to handle through traffic. Preventing backup paths from being used for normal traffic flow normally involves hiding them as long as the main (or normal) path is available. Floating static routes (see "Case Study: What's the Best Route?" later in this chapter), dial-on-demand circuits, and metric adjustments are good ways to hide a backup path until it's needed.