RFS: Possible network disconnect with primary database
standby database ID mismatch [0x99bb1983:0x8d17262b]

Symptoms

Standby alert log reports "RFS: Possible network disconnect with primary database"

At the standby DB side I get this error every minute.
RFS: Possible network disconnect with primary database
Standby database ID mismatch [0xAAAA:0xBBBBB]

Archive log transfer/apply works O.K.

Cause

The (error) message may be triggered by another database that  has log_archive_dest_2 pointing to standby instance of (error) message .

RFS process tries to communicate with the instance.

Solution

Check if you have DB with mismatched ID and its  log_archive_dest_X parameter value.
If found one but not in primary/standby config, disable log_archive_dest_2 to the instance, ie

From the error message of "Standby database ID mismatch [0xAAAA:0xBBBBB]"

Convert the hex number of first argument, 0xAAAA to decimal value using calculator

Then search db with the dbid.
SQL> select dbid from v$database;

SQL> alter system set log_archive_dest_2_state=defer;