Oracle Database 11gR2

Some points  in Oracle db 11g worth mentioning:

1. while trying to connect to the Oracle database as sys
SQL> connect /as sysdba
ERROR:
ORA-01031: insufficient privileges

or SQL> connect /nolog
    SQL> connected
    SQL>
    SQL>connect sys/passwrd@db11g as sysdba
ERROR:
ORA-01031: insufficient privileges

Solution:
Ensure REMOTE_LOGIN_PASSWORDFILE=EXCLUSIVE is set in the init.ora for the database

Ensure: linstner.ora file is pointing to actual Oracle Home

Ensure $ORACLE_SID is set at .bash_profile and .basrc

If still there is  an error, create the a password for sys user as follows:

$ orapwd file=$ORACLE_HOME/dbs/orapw$ORACLE_SID password=welcome1 entries=5

2. Oracle database unreachable during startup
SQL> connect sys as sysdba
SQL>password
SQL>welcome1
SQL>connected to an idle instance
SQL> startup
ORA-01078: failure in processing system parameters
LRM-00109: could not open parameter file '/u01/app/oracle/product/11.2.0/db_1/dbs/initDB11G.ora'
Solution:
There should be an existing spfile, like for example spfileDBID at $ORACLE_HOME/dbs

To resolve this problem execute the following: create pfile from spfile='/u01/app/oracle/product/11.2.0/db_1/dbs/spfiledb11g.ora';

You will get:

File created.

Then
SQL> startup
ORACLE instance started.
Total System Global Area 3140026368 bytes
Fixed Size                  2217464 bytes
Variable Size            1728055816 bytes
Database Buffers         1392508928 bytes
Redo Buffers               17244160 bytes
Database mounted.
Database opened.
SQL>