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 '\192.168.2.11testDF_CH00_%U';
ALLOCATE CHANNEL ch2 TYPE DISK FORMAT '\192.168.2.11testDF_CH01_%U';
ALLOCATE CHANNEL ch3 TYPE DISK FORMAT '\192.168.2.11testDF_CH02_%U';
ALLOCATE CHANNEL ch4 TYPE DISK FORMAT '\192.168.2.11testDF_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> RUN {
2> ALLOCATE CHANNEL ch00 TYPE DISK connect 'sys/oracle@realdb_1' FORMAT '\192.168.2.11testDF_CH00_%U';
3> ALLOCATE CHANNEL ch01 TYPE DISK connect 'sys/oracle@realdb_2' FORMAT '\192.168.2.11testDF_CH01_%U';
4> ALLOCATE CHANNEL ch02 TYPE DISK connect 'sys/oracle@realdb_3' FORMAT '\192.168.2.11testDF_CH02_%U';
5> ALLOCATE CHANNEL ch03 TYPE DISK connect 'sys/oracle@realdb_4' FORMAT '\192.168.2.11testDF_CH03_%U';
6> Backup incremental level 0 database;
7> RELEASE CHANNEL ch00;
8> RELEASE CHANNEL ch01;
9> RELEASE CHANNEL ch02;
10> RELEASE CHANNEL ch03;
11> }
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 ?