Building an OCS 2007 R2 Enterprise pool requires a back-end database running either SQL Server 2005 SP2 or 2008. As of R2, the recommended setup is the consolidated topology although the expanded configuration continues to be supported*. The database application can be 32- or 64- bit and the edition chosen is often dependent on the features required e.g. scalability or active/passive fail-over clustering.
A dedicated machine installed with SQL Server must already be present in order to receive the Back-End database. The latter will be automatically built when the R2 Enterprise pool is created. Since the Default Instance of SQL Server communicates over TCP port 1433 by default, you must explicitly configure Windows Firewall to permit this inbound traffic (to SQL) if the operating system you are running is base on Windows Server 2008. This statement applies to both SQL Server 2008 and SQL Server 2005.
You should closely follow the guide here or here to configure Windows Firewall for SQL Server 2005/2008 access. Failure to comply with this requirement will cause R2 setup to appear to hang and then terminate with an error (need to cancel without possibility to proceed further):
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Error
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Unable to retrieve the WMI setting of MSFT_SIPLogSetting from pool backend ’sqlservername’. Please ensure the backend database exists and is reachable.
(Error code: 0×800407D0)
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OK
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As most non SQL Administrators would agree, a word on how to correctly install SQL Server 2005/2008 for R2 is in order.
In addition to the SQL Server Database Services component, you should at least also select the Workstation components during setup. The latter is essential if you intend to perform administrative tasks on the Back-End server itself, such as creating new SQL instance via a GUI.
Should you start off by installing, say SQL Server 2005 RTM and then apply SP2 later, you will not be able to add new components at a later time if you only have the original RTM media. Keep this in mind when you install SQL Server using the instructions here.
For a brand new SQL Server 2005 setup as an example, consider choosing as a starting point to keep things simple …
… the Default instance (MSSQLSERVER)

… the built-in Local system account (service account)

… Windows Authentication Mode

… and the SQL collations settings to “Dictionary order, case-insensitive, for use with 1252 Character Set” (backward compatibility)

With the Back-End database installed and properly configured, the “Partial” warning should not even appear where you execute Step 1: Add Server to Pool**.
In other words, you should re-run this step if required until only “Success” is reported when you review the logs. All in all, this should pave the way for a less bumpy ride with subsequent steps in deploying R2 Enterprise Edition.
See also:
How to install Office Communications Server 2007 R2 to a SQL Server Named Instance using a non default TCP\IP port
* via command-line only
** Add Enterprise Edition Server to Pool
Technorati tags: LCS/OCS,
OCS 2007 R2, SQL Server 200x/2008