Traffic is routed to all of your available App Service instances. In the case when a zone goes down, the App Service platform will detect lost instances and automatically attempt to find new replacement instances and spread traffic as needed. If you have [autoscale](../app-service/manage-scale-up.md) configured, and if it decides more instances are needed, autoscale will also issue a request to App Service to add more instances. Note that [autoscale behavior is independent of App Service platform behavior](../azure-monitor/autoscale/autoscale-overview.md) and that your autoscale instance count specification doesn't need to be a multiple of three. It's also important to note there's no guarantee that requests for additional instances in a zone-down scenario will succeed since back filling lost instances occurs on a best-effort basis. The recommended solution is to create and configure your App Service plans to account for losing a zone as described in the next section.
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