Lets say that you have domain.com hosted with a 3rd party DNS provider and you would like to create GTM (BigIP-DNS) DNS load balancing by utilizing Sub-Domain Delegation.
In this scenario, there are 2 GTM. One in each DC (DC-1 & DC-2). The basic set up has been completed and the GTMs are in a common sync-group.
Create A-Records for the 2 GTM using their Listener IP addresses:
gtm1.wip.domain.com. IN A 100.100.100.100 gtm2.wip.domain.com. IN A 200.200.200.200
gtm1 and gtm2 exist in DC-1 and DC-2 respectively and 100.100.100.100 & 200.200.200.200 are the listener IP address configured on gtm1 and gtm2.
Delegate the sub-domain to the GTM using NS Records:
wip.domain.com. IN NS gtm1.wip.domain.com. wip.domain.com. IN NS gtm2.wip.domain.com.
Use CNAME records:
www.domain.com. IN CNAME www.wip.domain.com.
The above DNS records (A, NS & CNAME) will be added to the 3rd party DNS records that is hosting domain.com. Any request for
www.domain.com
will be sent to the 3rd party DNS provider which will then resolve to
www.wip.domain.com
because of the CNAME and that will be handled by the GTMs because of the NS & A records.