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1. The IAD registers with AS1 through

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   -3.
2. Subscriber is pre-assigned (based on Application Server subscriber data configuration) to AS1, thus

   Spacevars
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   10 has preset routing for the subscriber to AS1.
3. From the register, AS1 is informed that the subscriber is reached by

   Spacevars
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   -3.
4. The

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   -3 goes down and AS1 can not terminate the call to the subscriber until the subscriber re-registers.
5. The subscriber identifies that the

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   -3 is down only when it sends traffic (invite, refresh register etc).
6. IAD chooses an alternate IP address based on the DNS resolution and opens up a new path to the AS (paths should ideally be geo redundant).
7. When AS1 receives the terminating call it learns

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   -3 is unavailable and utilizes

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   -5 path. Hence IAD receives call despite failure of primary path.