BIP-IP DNS setting issue?

our dns server is not resolver a special domain 'www.airport.kr'

Below our dns server have a ns record 'a.www.airport.kr'. but not resolve a 'a.www.airport.kr'.

$ dig @dns www.airport.kr

; <<>> DiG 9.9.4-RedHat-9.9.4-18.el7 <<>> @dns221 www.airport.kr
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 32462
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.airport.kr.                        IN      A

;; Query time: 12 msec
;; SERVER: 
;; WHEN: 목  6월 16 18:34:03 KST 2016
;; MSG SIZE  rcvd: 43

$ dig @dns www.airport.kr +norecurse

; <<>> DiG 9.9.4-RedHat-9.9.4-18.el7 <<>> @dns221 www.airport.kr +norecurse
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28683
;; flags: qr ra; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 3

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.airport.kr.                        IN      A

;; AUTHORITY SECTION:
www.airport.kr.         967     IN      NS      a.www.airport.kr.
www.airport.kr.         967     IN      NS      b.www.airport.kr.

;; ADDITIONAL SECTION:
a.www.airport.kr.       791     IN      A       222.122.66.147
b.www.airport.kr.       791     IN      A       222.122.66.179

;; Query time: 0 msec
;; SERVER: 
;; WHEN: 목  6월 16 18:34:07 KST 2016
;; MSG SIZE  rcvd: 107

Generally big-ip dns is not have set a ns record?? (222.122.66.179 server is not have a.www.airport.kr record.)

[root@smlee:/root] $ dig @b.dns.kr www.airport.kr +norecurse

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.47.rc1.el6 <<>> @b.dns.kr www.airport.kr +norecurse
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 48175
;; flags: qr; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 0

;; QUESTION SECTION:
;www.airport.kr.                        IN      A

;; AUTHORITY SECTION:
airport.kr.             86400   IN      NS      ns20.airport.or.kr.
airport.kr.             86400   IN      NS      ns10.airport.or.kr.

;; Query time: 8 msec
;; SERVER: 61.74.75.153(61.74.75.1)
;; WHEN: Thu Jun 16 18:49:31 2016
;; MSG SIZE  rcvd: 81

[root@smlee:/root] $ dig @ns10.airport.or.kr www.airport.kr +norecurse

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.47.rc1.el6 <<>> @ns10.airport.or.kr www.airport.kr +norecurse
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 33732
;; flags: qr ra; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;www.airport.kr.                        IN      A

;; AUTHORITY SECTION:
www.airport.kr.         1800    IN      NS      a.www.airport.kr.
www.airport.kr.         1800    IN      NS      b.www.airport.kr.

;; ADDITIONAL SECTION:
b.www.airport.kr.       1270    IN      A       222.122.66.179
a.www.airport.kr.       1519    IN      A       222.122.66.147

;; Query time: 9 msec
;; SERVER: 116.84.243.2053(116.84.243.20)
;; WHEN: Thu Jun 16 18:49:41 2016
;; MSG SIZE  rcvd: 96

[root@smlee:/root] $ dig @222.122.66.179 www.airport.kr

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.47.rc1.el6 <<>> @222.122.66.179 www.airport.kr
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8873
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;www.airport.kr.                        IN      A

;; ANSWER SECTION:
www.airport.kr.         30      IN      A       116.84.243.135

;; AUTHORITY SECTION:
airport.kr.             500     IN      NS      B-GTM.BIGIP.COM.

;; Query time: 12 msec
;; SERVER: 222.122.66.17953(222.122.66.179)
;; WHEN: Thu Jun 16 18:49:57 2016
;; MSG SIZE  rcvd: 77

[root@smlee:/root] $ dig @222.122.66.179 a.www.airport.kr

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.47.rc1.el6 <<>> @222.122.66.179 a.www.airport.kr
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 42337
;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;a.www.airport.kr.              IN      A

;; AUTHORITY SECTION:
airport.kr.             60      IN      SOA     B-GTM.BIGIP.COM. hostmaster.B-GTM.BIGIP.COM. 15 10800 3600 604800 60

;; Query time: 9 msec
;; SERVER: 222.122.66.17953(222.122.66.179)
;; WHEN: Thu Jun 16 18:50:06 2016
;; MSG SIZE  rcvd: 96