force to use passphrase instead of password?

I have used openssh for a while. I found it is strange that sometimes it askes for passphrase and sometimes it asks for password even when I am logging into the same server with the same user account. Is there a way to force using passphrase?

The server runs openssh-3.1p1 on redhat 6.2. All the client machines are actually the same, redhat 7.3 with the same version of openssh.

Thanks in advance.

Jun Received on Thu Mar 20 13:33:15 2003