Question regarding DNS setting for Nighthawk(R) X4S R7800

Hello,

Currently I have 2 Piholes running on my network, and I have set them up as primary and secondary dns servers in my router settings. As mentioned in title, i have a Nighthawk(R) X4S R7800.

How does the DNS setting work in this instance? Is it balanced between the primary and secondary? or is it a failover situation (i.e. pihole1 goes down, traffic now goes to pihole2)?