Occasional Exceptions: "A network-related or instance-specific error occurred while establishing a connection to SQL Server"

Question

Occasionally our web application throws:

System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server) ---> System.ComponentModel.Win32Exception (0x80004005): The network path was not found

Now I have Googled maybe all kinds of variations regarding this error but all seem to point to the fact that the destination SQL server is either down or doesn't accept remote connections or something wrong with the connection string.

But my scenario is different, I can definitely reach the SQL Server (separate machine, same LAN) and this application runs normally most of the time, but in certain instances it starts to generate these errors. Then, apparently, they stop on their own then randomly they start again. Other applications on the same server to the same SQL Server (though different databases) throw these errors as well during the same period.

I have left a continuous ping between the two servers and I can confirm that during this period both servers have IP connectivity between them.

What could be the cause of this problem? What can I check next?

Answer 1

Did you make sure that TCP/IP is enabled under Sql Server Configuration management->SQL Server network config->protocols ?