Can I specify a backup LDAP server?

Yes, you can. But only if you're using the Active Directory Agent (with a Windows domain controller), or the Novell eDirectory Agent (with a Novell domain controller). You can enable this in System -> Authentication -> Enable/Disable. However, please review the R3000 Authentication User Guide to be sure you're familiar with the agents. Once you've enabled Active Directory Agent or eDirectory Agent, an option will be available in your LDAP domain's "Default Rule" page to configure a backup server.