I ran into a rather serious problem last weekend when attempting to upgrade one of my clustered servers. A previous update required me to reboot the servers in the cluster, and after the reboot the one server running SQL Server 2000 as a named instance wouldn't come back online under cluster administrator. I was able to bring it up using the Services admin tool, but in the Application error log it showed the following message:
[sqsrvres] checkODBCConnectError: sqlstate = 08001; native error = ffffffff;
message = [Microsoft][SQL Native Client]An error has occurred while establishing a connection to the server.
Searching the net led me to issues with DNS changes and the like, but nothing specific to my situation. We called Microsoft Support and explained the situation to Chetan Rao, a Microsoft SQL Server Support Engineer, and he referred to a KB article which described a problem with SQL Server Browser permissions on the SQL Server 2000 registry tree if you install SQL Server 2000 after installing SQL Server 2005. This wasn't our case, but we looked at the permissions settings anyway, and found that the Browser service account had all the necessary permissions in the registry.
During the conversation Chetan mentioned the possibility of adding the server names and port numbers as aliases under SQL Native Client Configuration. I wasn't able to do that during the business day because the server having the problem was a production server, and that I'd try it first thing in the morning.
In fact, that solved the problem. I was able to bring the server back online via cluster administration once the server was set up as an alias. I then moved the server group to another server in the cluster, and it didn't come online until I went into that server and added the aliases.
In a nutshell, if you've got both SQL 2000 and SQL 2005 servers in the same cluster, and the SQL 2000 servers are named instances, set up aliases in SQL Native Client Configuration on every server in the cluster to ensure proper failover. And a special thanks to Chetan Rao for helping us out of this jam.
Allen