An SAP client can either be configured to log on to a specific SAP instance or a logon group. If configured to log on through a logon group, SAP determines which running instance the client actually connects to.
If the instance to which the client is connected goes down, the client connection is lost and the client must re-log on. If the database is temporarily lost, but the instance to which the client is connected stays up, the client will be temporarily unavailable until the database comes back up but the client does not have to re-log on.
For performance reasons, clients should log on to redundant Application instances and not the PAS, ASCS or SCS. Administrators may wish, however, to be able to log on to the PAS to view logs, etc. After protecting SAP with LifeKeeper, a client login can be configured using the virtual SAP server name so the client can log on regardless of whether the SAP Instance is active on the SAP Primary or Backup server.