ORA-12528: TNS:listener: all appropriate instances are blocking new connections

Error:

bash-4.4$ sqlplus sys@standby as sysdba
SQL*Plus: Release 23.26.1.0.0 - Production on Mon Mar 16 11:04:30 2026
Version 23.26.1.0.0
Copyright (c) 1982, 2025, Oracle. All rights reserved.
Enter password: 
ERROR: ORA-12528: TNS:listener: all appropriate instances are blocking new connections 
Help: https://docs.oracle.com/error-help/db/ora-12528/

This error occurred on standby connection because standby database is in nomount state. Its means the standby database instance is in NOMOUNT or MOUNT state, so the listener blocks normal user connections. This is common during Data Guard creation with RMAN in Oracle Database.

For standby databases, you must allow SYSDBA connections even when the DB is not open.

Correct TNS Entry for Standby

In tnsnames.ora, add UR=A in CONNECT_DATA.

STANDBY =
 (DESCRIPTION =
   (ADDRESS = (PROTOCOL = TCP)(HOST = standby_host)(PORT = 1521))
   (CONNECT_DATA =
     (SERVER = DEDICATED)
     (SERVICE_NAME = ORCL)
     (UR = A)
   )
 )

UR=A means Use Remote SYSDBA connection even when instance is not open.

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