What does the UR = A in tnsnames.ora mean?

I recently had to connect remotely as sysdba to remote instances when I created Physical Standby Databases using RMAN.

There were some some connection problems along the way, and this was solved by setting the (UR=A) attribute in the tnsnames.ora entry for the database.

What exactly is the UR=A attribute and what does it do?

This is explained very well in a blog post from Ed Chen Logic so rather than explaining the same points all over again, continue reading there.

But in short, an instance (memory, background processes, but no data files) will inform the listener to block all connections to its corresponding service by default.

To bypass this restriction, you can add UR=A to the connect descriptor. From the primary server, I will then be able to connect to the standby instance running on prodserver2.oric.no.

Let's demonstrate it:

Start the standby database in nomount:

sqlplus / as sysdba
startup nomount
ORACLE instance started.

Total System Global Area 1.2885E+10 bytes
Fixed Size                  8635096 bytes
Variable Size            2986347816 bytes
Database Buffers         9865003008 bytes
Redo Buffers               24915968 bytes

Start a default listener on the standby server:

lsnrctl start

Make sure the default service is loaded: wait for a minute, or execute

alter system register;

Listener status is now:

Service "proddb01_stby.oric.no" has 1 instance(s).
  Instance "proddb01", status BLOCKED, has 1 handler(s) for this service...

Create a tnsnames.ora entry for connecting to the standby database from the primary server:

proddb01_stby.oric.no =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = proddb02.oric.no)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = proddb01_stby.oric.no)
    )
  )

Test it:

sqlplus sys/password@proddb01_stby.oric.no as sysdba

Result:

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

Add the UR=A to the connect_data section:

proddb01_stby.oric.no =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = proddb01.oric.no)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = proddb01_stby.oric.no)
      (UR = A)
    )
  )

And try again:

sqlplus sys/password@proddb01_stby.oric.no as sysdba

Result:

SQL*Plus: Release 12.2.0.1.0 Production on Fri Dec 9 16:43:09 2022

Copyright (c) 1982, 2016, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production

SQL> select host_name from v$instance;

HOST_NAME
-----------------
proddb02.oric.no

SQL> show parameter db_unique

NAME             TYPE      VALUE
---------------- --------- --------------
db_unique_name   string    proddb01_stby