What is the default when the DNS/GTM pool is down
The answer - like so many things - is it depends on the configuration of the GSLB pool and members, the DNS profile and the virtual server itself.
Both Pool members and pools have
as options, so they may provide resolution if the primary method fails.
If GSLB resolution fails, then the DNS profile options kick in ...
Use BIND Server on BIG-IP
allows the request to be passed to local bind - where results may be provided.
This may just round-robin the Wide-IP addresses and return results to services that are down, so you may need to disable this option
is there a DNS pool configured on the listener?
the request may reach this point and be passed to another DNS server for resolution ...
Unhandled Query Actions
this controls how the DNS profile responds to a query that has not been resolved