Primary DNS Fails how do we know 2nd will work?

I have 4 DNS servers if bind fails on one
How fast will it go to ns2 for a user?
How can I test this?
Who is Participating?
If Resolver/cache/cachingNameServer, finds the first NS fails (timeouts) goes to next NS. It's a matter of seconds.
Also note, that for resolver there is no such thing like primary NS.
If You have four NSes serving same domain/zone, they should be asked same times(say load balancing).
It's because every time someone(resolver) asks for list of nameservers serving particular zone, it gets the list in random order (usually in RR manner).

> How can I test this?
Just fail one of NSes, Translations should be still visible.
Gabriel OrozcoSolution ArchitectCommented:
it will try the next dns only when the first timeouts.

this is bad, but it is how it works.
of course, as ravenpl says if you have listed many dns servers for a domain then they will be asked using round-robyn (load balancing), so if one of four servers is down, 25%  of your users will experience timeout-then-works kind of service.
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