It was working like this, in /etc/openldap/ldap.conf
URI ldap://127.0.0.1/
BASE dc=example,dc=com
TLS_REQUEST never
TLS_CACERTDIR /etc/pki/tls/certs
I was able to connect our secured ldap server.
Because the heartbleed issue, I install openssl to 0.9.8e-fips-rhel5. Now I have "unable to bind to server: Can't contact LDAP server".
Update
Thanks alxgomz for picking up the typo. The typo means my server is always using certificate to verify secured ldap servers. The message "unable to bind to server: Can't contact LDAP server" is not helpful in this case, because it doesn't tell you why I am not connecting. I actually get a useful message by doing ./path/to/ldapsearch -H "ldaps://xxx.com"
The lesson is to use different ways to get a better diagnosed message.