I would like to ask for some clarification on the following situation / Scenario.
Let's assume I have a Domain, whose DNS A Records are registered by means of 2 Pairs (4 in total) of Name Servers; each Name Servers Pair is hosted within a different datacenter, for redundancy purposes.
In case of loss of connectivity towards one of the Pairs of Name Servers (due to whatever reason, like Internet Firewall outage, or data center outage), and assuming that the other pair of Name Servers hosted in the second location would still be online and accessible from the internet, would this affect the DNS resolution process for external clients and, if so, to what extent ?
My understanding is that the Recursive DNS Servers would be querying the Root Servers and all 4 NS Records for the zone would be initially returned. What would the Recursive DNS Servers than do during the next step if 2 of the NS could not be reached and the query for the A Record sent to those Servers were to time out ?
Could the resolution Process get significantly affected or even technically fail on the client side ?
Or would the Recursive DNS Resolver just proceed through the NS list and query the remaining Name Servers which are online, eventually successfully resolving the DNS Record ?
Thanks for clarifying.