Im tying to figure why would we setup a Cluster of DNS arround the world. Is it simply for redundance ? if one server crash other still up to respond request. that's all ? or having many dns server in many country increase site speed or anything ? Wouldn't be better to have a cluster for your apache server ? likely Network Load Balancing ? so how to create a cluster for apache server ?
Let say your have three server, but want to use ns1.domain.com and ns2.domain.com as dns adress for all three server. With no dns clustering, you would have to have different dns adress for all server. With dns clustering, you can have a pair of dns adress for all your servers in the cluster. You are also right about redundancy. If you have many server physically seperated, you could have a dns1 on one server, and dns2 on another server. So if server1 go down, dns are still active because it is likely that server2 is also down. As for apache cluster, you shoudl use a physical laod balancer. Much like a router, that distribute the visits across the number of server pluged in the load balancer.
I'd bet those who need DNS clustering, have a need for co-lo facilities spread across the world and it just happens they have a server that does DNS as well. seriously doubt anyone outside of domain companies would just have DNS servers. I mean it sounds good and everything but the accounting dept. would balk.