LTM discards DNS responses with ECS option

Hi all,

 

I am using BIG-IP 12.1.3 node to configure Virtual Server with pool of UDP DNS servers, and noticed strange behavior - if I configure VS "DNS Profile" from None to dns, it stops serving DNS queries with EDNS0 ECS option. From network traces I can see that request is forwarded to pool member, which responds with valid DNS answer, which then silently discarded by LTM. Requests with no ECS option are served fine.

 

Any suggestions on why it's happening and how to make it work ?

 

BR