Designing Link RedundancyIt is often necessary to provision redundant media in locations where mission-critical applicationtraffic travels. In Layer 2–switched networks, redundant links are permitted as long as STP isrunning. STP guarantees one, and only one, active path within a broadcast domain, avoidingproblems such as broadcast storms (when a broadcast continuously loops). The redundant pathautomatically activates when the active path goes down.Because WAN links are often critical pieces of the internetwork, redundant media are oftendeployed in WAN environments. As is the case in Figure 3-20, where a Frame Relay circuit is usedin parallel with a backup IPsec connection over the Internet, backup links can use differenttechnologies. It is important that the backup provide sufficient capacity to meet the criticalrequirements if the primary route fails.
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