Record MultiSubnetFailover value at connection?

Question

With AlwaysOn, there are 3 options for the keyword MultiSubnetFailover

• True (yes)
• False (no) (example)
• Not sent/used

Not all applications support it's use in connection strings. Not all vendor contacts understand it's importance to performance at failover.

As a DBA I would like to be able to validate what property is being used by the application connection. If I have not made the appropriate setting change to RegisterAllProvidersIP it can cause issues. See: Follow-up: After Creating an Availability Group Listener

It does not appear to be capture in either sys.dm_exec_sessions nor sys.dm_exec_connections

How can I capture what (if any) keyword values were used to create a connection to one of my instances?

Answer 1

I don't believe this configuration setting is sent to the server. It controls the client's interpretation of a DNS lookup returning multiple IP addresses, and there is no functional need for the server to be informed about it. And the client doesn't send arbitrary name/value pairs to the server. The fields that a client sends to the server are dictated by the TDS protocol, and it would require a protocol revision to add new ones.

Also while properly setting MultSubnetFailover at the client continues to be a best-practice, newer SQL Server client libraries have changed their behavior to mitigate the issues arising from not setting MultiSubnetFailover. See Using Transparent Network IP Resolution. Newer clients will have only a brief delay in connecting when the first IP returned from DNS is not the active AG Listener, instead of the old behavior of waiting 20-30 seconds before attempting to connect to the second IP.