On a Windows Cluster it is possible to install a SQL Server Cluster, which will be a separate entity with disctinct Virtual Server Name, IP address, and pipe name. The nodes of the SQL Cluster must be able to communicate with one another. This communication may fail if the TCP port on which SQL Server is listening is not excepted from firewalls on the machines. This port can be found by using the Configuration Manager utility by viewing the properties of the TCP protocol or by checking the SQL Server errorlog, typically located at "Program FilesMicrosoft SQL ServerMSSQL.xMSSQLLOG". Here MSSQL.x is the instance ID of the clustered instance in question.
If the Cluster instance being installed is a named instance, a service called SQL Browser is used in order to figure out the port or pipe SQL Server is listening on. For SQL Server Cluster installations, this service is running by default. If this service is not running, it will not be possible to communicate with the remote nodes. To manipulate this service, open services.msc, find SQL Browser, and make sure its state is Automatic and Running. SQL Browser listens on a UDP port, which must also be excepted from firewalls on the machines. Even if SQL Browser is running, in case any of the nodes has firewall ON, it is necessary to add exceptions for UDP port 1434.
These issues with SQL Browser (service not running or TCP and UDP port not excepted from firewall) can also block remote connectivity to non-clustered instances.
More about Windows Firewall impact on APIs for SQL Server enumeration can be read at http://blogs.msdn.com/sql_protocols/archive/2005/09/24/473502.aspx. More about Firewall impact on SQL Server Cluster can be found at http://blogs.msdn.com/sql_protocols/archive/2006/02/27/unable-to-connect-to-a-sql-server-named-instance-on-a-cluster.aspx.
Stoyko Kostov, SQL Server Protocols
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