ORA-12545 error is seen in a a Oracle agent alarm
ORA-12545 error occurs when the client is not able to contact the listener on the node specified. For example: when the listener has not been started on the node, if the node address has been specified incorrectly, or a firewall blocks access.
This could be an intermittent or transient alert such as when a failover occurs.
You can trace this using a few Oracle tools:
Ensure the local_listener in the spfile/tnsnames is set.
heck for load_balance=yes set in your tnsnames.ora file. Did a listener failover occur?
OS specific DNS commands