Why ldap sasl digest-md5 only works for clear password?

[Ming Deng <mingd@oeone.com>]

Rpm version: openldap 2.1.16

I want to authenticate users again ldap server with Digest-MD5 SASL 
mechanism. If I store user password in clear text format in userPassword 
attribute of ldap directory, it works fine. But if I store the password 
in any hashed format. e.g. MD5, SHA, it will fail with:
ldap_sasl_interactive_bind_s: Invalid credentials (49)
      additional info: SASL(-13): authentication failure: client 
response doesn't match what we generated

Thanks,

Ming