Client connection timeout after switchover to standby

Hi,

We have two data centers and all connection strings have the similar as below:

ALIASNAME =

  (DESCRIPTION =

       (ADDRESS = (PROTOCOL = TCP)(HOST = primary-scan)(PORT = primary.port))

       (ADDRESS = (PROTOCOL = TCP)(HOST = standby-scan)(PORT = standby.port))

       (CONNECT_DATA =

           (SERVICE_NAME = database_servicename)

       )

  )

When we complete the database switchover, clients can still connect to database that is running on new primary.

However, we got the problem that when the primary nodes (all nodes on the same RAC) are power off, client connection will timeout after (15 seconds * number of nodes).

It looks like client still PING all available nodes on the first provided address (primary-scan as above), then it will go to PING second address.