There is no sane condition in Redis that will make it not replying at all AFAIK, even if you set maxclients to 1 the next clients will have an error returned (and the connection closed ASAP). But yes, it is important to understand what down means. I think one of the safest things to do is "down == unreachable". So if you don't get any reply at all, for the configured amount of time consecutively, the server is down. And of course the other redis-sentinels have to agree for the fail over to start.