What happens when the primary server is not reachable?Nope, unless if you want them to. Then, you will have to reverse-update your primary server. Basically what you do is have the primary server running and the rest is all caching naming servers. Ideally, the secondary and tertiary should be located in different subnet from primary as well as geographical region (although I know it's hard to do, like I said "ideally".).Should client send the update record to secondary DNS Server in that case?