When does 2nd DNS server kick in?

I am trying to understand more about DNS and my questions below don't refer to any domain in particular.

Assuming we have a domain with 2 name servers (1 primary, 1 secondary), both are properly configured as master/slave and are hosted at different geographical locations.

My questions are:

1. From users end, do they always look up the primary server by default or randomly choose any of the NS? What is the default behaviour of popular OSes - Windows, Mac, Linux etc?

2. If the primary is down, does it automatically failover to the secondary? Is there any chances that some clients/dns servers do not retry on secondary? Is there any time gap between the switch?

The reason I am asking is that despite DNS redundancy, I have seen on a few occasions that some clients does not automatically failover to secondary when the primary is down. This occurs at users end, their ISP dns servers insist on using the primary.

Any idea? Thanks.

 

 

 

 

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