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Lets say I forget to update the SOA serial when updating the zonefile. Of course, this means any slaves will not sync to the master because it thinks no change has been done.

The question I have, is, does SOA serial have any effect on ISP caching nameservers?

Eg, lets say a customer at ISP does a request for A www.example.org and get 93.184.216.34 . Then I change the domain to have IP 93.184.216.35, but forgets to update the SOA serial.

When TTL expires, and customer's ISP caching resolver does a new request for www.example.org and sees 93.184.216.35, will it refuse to update the cache and still serve data out of the cache, if the serial is not incremented?

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No. the SOA serial is used for zone transfers. Some times you find yourself in the need to reset the serial, say if you accidentally put in a too high number or something like that. It would be very inconvenient if the name servers querying your DNS stopped updating because of that.

You can easily test this yourself.

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