I thought that this would be interesting to post as I don’t think that many people are aware of this. So we all know that when the OAB is generated it will be dropped to the local OAB share on the OAB Generation server in the following location: [D:\Program Files\Microsoft\Exchange Server\ExchangeOAB\<OAB Guid>]. By default we will always look to this location for generating later versions of the OAB.
What about a fail safe mechanism?
Now if you happen to have a OAB Version 4 public folder we can find this to be a great resource. In the event of a problem (Examples: Files in the OAB share get plucked out by virus software, they get deleted, drive gets nuked, etc), we will fall back and resort to the public folder as a backup. Since we can no longer have access to use the files in the OAB share to construct the OAB, we will connect to the public folder store and download those files so we can reconstruct the new OAB files for the OAB share. This will stop us from having to make the initial query to the domain controller to re-query for the data, AND rebuild the OAB. This ensures that we retain the same Guid as before. This will stop the generation of a new OAB with a different Guid which causes full downloads.
So, before you get rid of all of your public folder servers, it might be a good idea to keep one around