BIG-IP DNS ACTIVE-ACTIVE Sync Group Traffic Flow

> Out of the 2 Active-Active Systems, to which BIG-IP DNS, the LDNS will send the query for subdomain.

That depends on the LDNS. You cannot control that.

Your main DNS server replies to a lookup for gtm.yourdomain.com by saying

that the names-servers for gtm.yourdomain.com are ns1.gtm.yourdomain.com and ns2.gtm.yourdomain.com, and by the way, I'll give you the IP addresses for ns1.gtm.yourdomain.com and ns2.gtm.yourdomain.com (glue records) as well.

The LDNS now has two name servers and IP addresses for them - it then selects one (or maybe both) and sends the next step in the recursive lookup to get the authoritative SOA from the child domain name-severs and the authoritative name-servers from SOA record.

But what the LDNS does then is entirely implementation dependent. It may select one nameserver randomly every time. If it caches the result, it may use a round-robin scheme. It may query both and deliver the fastest returning result. It might always use the first one. But however it does it, it is out of your control.

For a good summary of how this works and the possible (and actual) issues, look at

Optimizing DNS Server Selection