SQL Server connection string question

Answer 1

At a guess the 2005 and 2008 instances would be listening on different ports. Perhaps you need to set the connection string for the SQL Server 2008 instance to the correct port; you could be hitting the SQL Server 2005 listener and asking it for an instance that is not present on the 2005 installation.

What port is the SQL Server 2008 instance listening on - This article discusses the subject. and has a powershell script that actually interrogates the system to find out what port the DBMS is really listening on. It also talks a little about where this config lives in the registry.

Answer 2

Copied from my answer on your question on ServerFault...

The connection strings have 2 differences so I'll try to answer for this...

• Named instances do not listen on port 1433 (only the default instance by, er, default)
• The instance name is ignored when the port is specified

So:

• ServerName\InstanceName,1433 means connect to server "ServerName" on port 1433
• ServerName\InstanceName means connect to server "ServerName" and resolve instance name to correct port (resolve uses port 1434 UDP)

However, network library=dbmssocn means use tcp. It could be the SQL Server instance (see port stuff) does not listen on tcp