BIND9 Notifications

In your bind configurations when a master DNS server makes an update it looks inside the zone file, and any DNS servers listed inside are sent a notice saying there has been an update. After this the slave queries the SOA for updates. If you happen to have an additional DNS server that needs to be notified but isn’t in this list, there is an “also-notify” option.

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options {
....
    also-notify {10.1.0.15; 172.28.32.7;}; // all zones
....
};

Also-notify will do just that…send a notification over to these servers that there has been a change.

In your master you will also need to allow transfers from any additional servers, otherwise all of this is moot.