RMAN backup of RAC database failing Backup goes through if I don’t have any explicit connection to any instance : 

RUN {
ALLOCATE CHANNEL ch1 TYPE DISK FORMAT '\\192.168.2.11\test\DF_CH00_%U';
ALLOCATE CHANNEL ch2 TYPE DISK FORMAT '\\192.168.2.11\test\DF_CH01_%U';
ALLOCATE CHANNEL ch3 TYPE DISK FORMAT '\\192.168.2.11\test\DF_CH02_%U';
ALLOCATE CHANNEL ch4 TYPE DISK FORMAT '\\192.168.2.11\test\DF_CH03_%U';
Backup incremental level 0 database;
RELEASE CHANNEL ch1;
RELEASE CHANNEL ch2;
RELEASE CHANNEL ch3;
RELEASE CHANNEL ch4;
}


But it fails when I specify explicitly the instance name : 

RMAN&gt; RUN {
2&gt; ALLOCATE CHANNEL ch00 TYPE DISK connect 'sys/oracle@realdb_1' FORMAT '\\192.168.2.11\test\DF_CH00_%U';
3&gt; ALLOCATE CHANNEL ch01 TYPE DISK connect 'sys/oracle@realdb_2' FORMAT '\\192.168.2.11\test\DF_CH01_%U';
4&gt; ALLOCATE CHANNEL ch02 TYPE DISK connect 'sys/oracle@realdb_3' FORMAT '\\192.168.2.11\test\DF_CH02_%U';
5&gt; ALLOCATE CHANNEL ch03 TYPE DISK connect 'sys/oracle@realdb_4' FORMAT '\\192.168.2.11\test\DF_CH03_%U';
6&gt; Backup incremental level 0 database;
7&gt; RELEASE CHANNEL ch00;
8&gt; RELEASE CHANNEL ch01;
9&gt; RELEASE CHANNEL ch02;
10&gt; RELEASE CHANNEL ch03;
11&gt; }
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-12001: could not open channel ch00
RMAN-10008: could not create channel context
RMAN-10003: unable to connect to target database
ORA-12154: TNS:could not resolve the connect identifier specified


Any ideas ?
Are you able to tnsping realdb_1, realdb_2 individually? 

If you're using SCAN, but haven't manually added node specific entries to your tnsnames.ora, this problem is likely to occur upon requesting connection to a specific node, since SCAN may be configured to resolve to the service_name of the RAC, rather than to the "instance_name".

Here's my tnsnames.ora:

USAPROJ =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = rac-scan.localdomain)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = usaproj.localdomain)
    )
  )

USAPROJ1 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = rac-scan.localdomain)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = usaproj.localdomain)
      (INSTANCE_NAME = usaproj1)   &lt;-------
        )
  )


I created another entry, usaproj1, and in addition to service_name parameter, I added "instance_name", to specify the node I wanted to connect. 

Hope it helps.