Fatal NI connect error 12541

If you see this error message in the alert log file of your primary database in a dataguard environment, it is most likely that listener on the standby database host(s) is not running. As a result, primary database will not be able to ship redo data to the standby database. This failure of log shipping will add lines similar to the following in the alert log file of the primary database.

Fatal NI connect error 12541, connecting to:  (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=172.125.16.100)(PORT=1521))(CONNECT_DATA=(SERVER=DEDICATED)(SERVICE_NAME=OPERA)(CID=(PROGRAM=oracle)(HOST=MYDBHOST)(USER=oracle))))   VERSION INFORMATION:         TNS for Linux: Version 11.2.0.3.0 - Production         Oracle Bequeath NT Protocol Adapter for Linux: Version 11.2.0.3.0 - Production         TCP/IP NT Protocol Adapter for Linux: Version 11.2.0.3.0 - Production   Time: 29-JUN-2017 01:21:38   Tracing not turned on.   Tns error struct:     ns main err code: 12541 TNS-12541: TNS:no listener     ns secondary err code: 12560     nt main err code: 511 TNS-00511: No listener     nt secondary err code: 111     nt OS err code: 0

To solve this issue, make sure that listener on the standby database host(s) is running and is able to accept in coming connections from the primary site. Once listener is started, use tnsping or sqlplus session from the primary database to connect to the the standby database to confirm whether connection is made successfully.