Need help for configuration - LDAP with custom files Failover

Dear All, I'm facing a problem with Freeradius 2.0.4. I want to make a configuration to allow our Cisco routers to auth via RADIUS. For this, we're using a password file now (let's call it ciscopwd) and another file for granting rights. I want to change the config file, so that the auth is done by LDAP (plain passwords), BUT if the LDAP is not available the ciscopwd file will do the job. How can this be done? (moreover, I want to have some LDAP group limitations on the logins) Thanks in advance, Tamas