Not all load balancers support this (I recall some very expensive alteons that didnt) but worse is when the LB does support it but it isn't configured to check.

Not all load balancers support this (I recall some very expensive alteons that didnt) but worse is when the LB does support it but it isn't configured to check. Out of the box most LB's will do a TCP bind and if that works, then fire away. The reverse of this issue can also cause problems where you get a bad node that accepts the request but never returns (or takes 2-3 mins to respond). As a rule the timeouts will cause queuing, thread pool starvation and general not working all the way back up your request chain to whatever is facing the internet where your site will either hang or give back a 503 page.