Multiple LDAP servers can be configured for authentication. Once all the LDAP servers to authenticate against have been added, they can be further organized into server groups with Primary and Backup servers, using the options under the LDAP pane of the LDAP servers dialog:
If more than one LDAP server is configured the first server in the list is used for authentication. If that server fails or is disabled, the next server in the list is tried and the process continues until a valid server is found or the list is exhausted.
The next server in the list is only tried if the current server cannot be used. If a server is valid but the login fails due to an invalid user name or password, no further authentication attempts are made on the other servers.