Queries regarding the working behaviour of isFallback in Apigee LoadBalancing

[Aravind]

<TargetEndpoint name="default">

 <HTTPTargetConnection>

      <LoadBalancer>

        <Algorithm>RoundRobin</Algorithm>

        <Server name="target1" />

     <Server name="target2">

          <IsFallback>true</IsFallback>

        </Server>

      </LoadBalancer>

      <Path>/test</Path>

  </HTTPTargetConnection>

</TargetEndpoint>

Above is the isFallback snippet and below are my questions on it 

1. If target 1 failed, request will route to target 2 within the same call or from subsequent (from 2nd call) requests will start routing to target 2?

2. If it tried within the same call, will latency will increase? bcoz request tries with target 1 and failed and tried with target 2.

3. And how requests know when it starts routing to target 1 when it was up and running.

4. What error scenarios it will consider as a failure and choose the fallback?

Thanks in adavance.