EXCHANGE 2013 OAB

Hi,

Needed some clarfications can you please help ne

1: in exchange 2013 what is HAB?
2: How we can apply an address for an ou
3: how we can rebuilt oab ? say for i.e in one server holding an oab is down and another sit oab were not getting updated. how we can over come this

Thanks
Muthu

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Thanks Muthu