You may also have DNS resolution from a standby F5 device in a cluster if the DNS listener is a local self ip :
https://support.f5.com/csp/article/K5427
A BIG-IP pair in which both units are licensed with BIG-IP DNS or BIG-IP Link Controller and both units answer DNS queries regardless of the BIG-IP active-standby state
On each unit you can configure a listener to use the non-floating self IP address or other local IP address on which the system listens for connections. This configuration allows either unit to respond to DNS queries regardless of the BIG-IP active-standby state.