Theory

By default OAB is a point in time snapshot of global address list and it is used as cached source of information about Exchange recipients properties. OAB is stored on Exchange servers (see my previous article http://ficility.net/2013/03/04/oab-differences-between-exchange-2010-and-exchange-2013-in-brief/) and downloaded to client once Outlook is configured in Cached mode. I would like to test modes of using address book,
while Outlook is in cached mode. There are several methods to download OAB. These methods depends on registry settings of Outlook (full article here:http://support.microsoft.com/kb/823580)
shortly:

If the following registry key is present (XX.0 means office version – 15.0 for Office 2013), Outlook behaves upon the DWORD value inside:

HKEY_CURRENT_USER\Software\Microsoft\Office\XX.0\Outlook 
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\XX.0\Outlook

DownloadOAB DWORD supported values:
  0 = The Offline Address Book does not download automatically.
  1 = The Offline Address Book uses the Download Full Items download mode. This is the default setting.
  2 = Download the Offline Address Book in any download mode, but download a differential update in the Header only download mode.
  3 = Always download the Offline Address Book and a differential update in one of the following download modes:
Download Headers and then Full Items
Download Full Items
Download Headers

The goal of this article is to test differences in OAB behavior between modes 0 and 1. I want to see how it looks when:

1) Mode 1 – The Offline Address Book uses the Download Full Items download mode.
a)Download OAB and check
b)Change GAL and test downloading OAB instantly
c)Update OAB and check

2) Mode 0 – The Offline Address Book does not download automatically.
a) Test behavior once mode 0 is configured while old OAB files are still on the client
b) Test behavior once mode 0 is configured while old OAB files are removed from client

Test scenarios

1) Mode 1 – The Offline Address Book uses the Download Full Items download mode.

By default OAB is downloaded from server hosting Active mailbox database with Organizational mailbox. (or in 2010 from CAS server distribution point or in 2007 and 2010 from Public Folders)

a) Download OAB and check

I have opened OAB from my mailbox and result is in the Picture

b) Change GAL and test downloading OAB instantly

Creation of a mailbox doesn´t updtate OAB itsetf. To create mailbox use command

New-Mailbox OAB_Test_o1 -UserPrincipalName OABTestO1@salonovi.cz
WARNING: A script or application on the FRONTEND1.SALONOVI.CZ remote computer is sending a prompt request. When
prompted, enter sensitive information such as credentials or password only if you trust the remote computer and the
application or script requesting it.

cmdlet New-Mailbox at command pipeline position 1
Supply values for the following parameters:
Password: ********

Name                      Alias                ServerName       ProhibitSendQuota
----                      -----                ----------       -----------------
OAB_Test_o1               OABTestO1            backend1         Unlimited

c) Update OAB and check

To update OAB use command

Get-OfflineAddressBook | Update-OfflineAddressBook

Now I have tested if OAB change is reflected in client computer (should not be)

And now I have downloaded new OAB to client and tested again

2) Mode 0 – The Offline Address Book does not download automatically

One registry key setting and Outlook client will work online from Addres book point of view, BUT! This setting also requires to clean up OAB files from client computer to behave correctly. I will test both possibilities and try to find differneces in behavior.

First I will set up registry key and restart Outlook

In my lab I dont use any special GPO so yhe setting is done via registry key HKEY_CURRENT_USER\Software\Microsoft\Office\XX.0\Outlook

OAB files are left on the client

a) Test behavior once mode 0 is configured while old OAB files are still on the client

New mailbox should be immediately visible for client without need to download OAB since information should be available online.

b) Change GAL and test if it appears immediately to client

Change in OAB is not visible immediately, because we have OAB files on the client computer and Outlook use those!

New-Mailbox OAB_Test_o2 -UserPrincipalName OABTestO2@salonovi.cz

c) Update OAB automatically by restarting Outlook

If Outlook is restarted, OAB version is checked / downloaded from Exchange server.

AND NOW THE NAUGHTY stuff! OAB is not updated anymore even you download files successfully from Exchange. In this stage Outlook is stuck somewhere between mode 0 and 1 and updates are not received by client.

d) Update OAB and download change to client manually even mode 0 is used

In this scenario I would like to prove, that setting mode 0 is not the only thing to consider to have Outlook work correctly.

• To update OAB use command

Get-OfflineAddressBook | Update-OfflineAddressBook

• Download OAB to client and check if changes are displayed to client.

 Changes are only reflected if I try to manually download full or incremental copy of address book. OAB is not downloaded during the client startup!

OAB files are removed from client

a) Test behavior once mode 0 is configured while old OAB files are removed from the client

I added new mailbox again, updated OAB, but for now I have removed all OAB-Related files from client. Changes should appear to client immediately.

To remove OAB files

• locate:
  c:\Users\<USERNAME>\AppData\Local\Microsoft\Outlook\Offline Address Books\
• Delete folders
  Example: (c:\Users\lelicek\AppData\Local\Microsoft\Outlook\Offline Address Books\6a285982-48d6-43ee-979b-f84dd5b7d989\)
• Start Outlook
• After Outlook startup, folders will be re-creaded, but will be empty. It proves we have mode 0 set to Outlook client.
• open Offline Address Book
• Newly created mailbox should be here

 d) Do change in GAL and test if it appears immediately to client

• Create new mailbox

New-Mailbox OAB_Test_o5 -UserPrincipalName OABTesto5@salonovi.cz

• Open Offline Address Book and new mailbox should be again there

e) Download OAB manually and test if the changes made to GAL after OAB download will be imediatelly visible to client

Conclusion

Mode 1

• Conclusion is , that mode 1 works fine and as it should and user will get updated OAB after regularOAB update schedule or after manual run of Update-OfflineAddressBook command.
• Note that in Exchange 2010 you must restart File distribution service, to distribute updated OAB to WEB distribution points. In Exchange 2013 it is not needed anymore.

Mode 0

• Use mode 0 only in case, that you do a lot of changes in GAL and you need clients to see changes immediatelly while taking advantages of Outlook Cached connection.
• Once mode 0 is used, administrator has to make sure, that OAB files will be removed from client computer (for example by logon script / GPO), otherwise user must use manual OAB update via Send/Receive -> Send/receive groups / Download Address Book.
• If user tries to manual download OAB while mode 0 is used, Online functionality will STOP working from that time until OAB files are deleted again!