Question about AD server restart

We are using the Active Directory LDAP integration. We need to perform some server maintenance on one of our domain controllers. Currently, that is the server that OpenFire is authenticating against. Is there a way to put in a failover AD server for authentication? What kind of impact will there be on the users in Spark when the server is restarted. We are a 24-hour call center, so there isn’t really a time when no one is logged in. Thanks in advance.