Whenever a DB2® server or DB2 Connect server crashes, each client that is connected to that server receives a communications error which terminates the connection resulting in an application error. In cases where availability is important, you should have implemented either a redundant set up or the ability to fail the server over to a standby node. In either case, the DB2 client code attempts to re-establish the connection to the original server which might be running on a failover node (the IP address fails over as well), or to a new server.
These commands update the alternate server information for a database alias in the system database directory.