linux system password-free login

There are two machines, both systems are CentOS6.5, and the IPs are 192.168.2.150 and 192.168.2.151 respectively. Now 150 requires SSH password-free login to 151.
Execute the command on 150, the current login user is root:
# ssh-keygen -t rsa
# ssh-copy-id -i ~/.ssh/id_rsa.pub [email protected]