why this query cause ServFail

[Hillary Nelson]

We've been seeing sporadic failure of resolve this name
web1.production.tacc.utexas.edu from our nameserver.

There are 6 NS listed for domain production.tacc.utexas.edu, two of the six
don't seem to work(dc1.production.tacc.utexas.edu 129.114.13.17 and
dc2.production.tacc.utexas.edu 129.114.13.18).

If our nameserver hits the two and doesn't get any response, it sends
'ServFail' to client, shouldn't the our nameserver keeps trying the other
four working nameservers listed for the domain ?

Here is the tcpdump:

12:33:38.593146 IP 10.79.1.6.51980 > 192.168.1.100.53: 60950+ [1au] A?
tas.tacc.utexas.edu. (48)
12:33:38.593573 IP 192.168.1.100.54985 > 129.114.13.18.53: 40455% [1au] A?
web1.production.tacc.utexas.edu. (60)
12:33:43.593131 IP 10.79.1.6.51980 > 192.168.1.100.53: 60950+ [1au] A?
tas.tacc.utexas.edu. (48)
12:33:47.593796 IP 192.168.1.100.49009 > 129.114.13.18.53: 38559% [1au] A?
web1.production.tacc.utexas.edu. (60)
12:33:48.593234 IP 10.79.1.6.51980 > 192.168.1.100.53: 60950+ [1au] A?
tas.tacc.utexas.edu. (48)
12:33:48.593583 IP 192.168.1.100.53 > 10.79.1.6.51980: 60950 ServFail 0/0/1
(48)


Thanks in advance for your help!
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