which NS record will be cached?

[MontyRee]

Hi, all.


I have one question about NS cache ttl.
for example, I can get two different NS TTL like below.  

$ dig  google.com ns +trace

google.com.             172800  IN      NS      ns2.google.com.
google.com.             172800  IN      NS      ns1.google.com.
google.com.             172800  IN      NS      ns3.google.com.
google.com.             172800  IN      NS      ns4.google.com.
;; Received 164 bytes from 192.5.6.30#53(a.gtld-servers.net) in 173 ms

google.com.             345600  IN      NS      ns4.google.com.
google.com.             345600  IN      NS      ns1.google.com.
google.com.             345600  IN      NS      ns2.google.com.
google.com.             345600  IN      NS      ns3.google.com.
;; Received 164 bytes from 216.239.34.10#53(ns2.google.com) in 43 ms

so, on resolving DNS, which NS record TTL will be cached generally?
172800 or 345600?


Thanks in advance.