It will keep going between the two. If you want it to use a primary unless it is unavailable, you would pick failover.

As for if both are unavailable, there is a configuration option on the listener that allows you to accept if ldap is unavailable. You will see your work queues grow until the ldap returns.

The ldap is cached if they are already in the list it knows about. You can setup information on how many entries to cache and for how long. Under ldapconfig->edit->pick the ldap server configuration->server->cache.