Creating a key with the defaults. If you enter a passphrase... you will have to enter it every time you use the key.
[user@server1]# ssh-keygen -t rsa -b 2048
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): # Hit Enter
Enter passphrase (empty for no passphrase): # Hit Enter
Enter same passphrase again: # Hit Enter
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
Copy the key to the remote server...
You will have to log in this time but not he next.