GTM - Multiple name server

Hi,

So do you have a cluster with a floating VIP?

Or each BIG-IP has its own interface and you are syncing just the DNS Config?

If clustered the IP will just move over and the service keeps up.

If they are independent devices with sync'ed config but both available to the dns client on the endpoint or global dns servers, when one stops responding it'll be marked down by the server and the other one answers.

Also consider, if you are running it as a cluster and one device is in the DC and the other is in the DR.

The VIP is i guess in the DC and only moves over tot he DR if the DC goes down.

But if you have 2 independent devices one in DC and one in DR, the DNS service isn't setup as Primary / secondary both servers will be able to respond to DNS as all times. It's the client side that works out which one to use.

You may notice for example on your windows machines that even though you put 2 DNS servers in to the config, both of them are being used for DNS resolution,

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.

In short, the external user DNS resolver should try to reach ns1.test.ae to resolve DNS query. 

When DNS query sent by a DNS client to a DNS server is down, the DNS client times out. Once it times out, it will reach out to one of the other name servers listed in zone. That's why we require at least minimum of two DNS Name Servers.