GTM HA / Config synch

GTM is always active - there's no concept of failover/standby within the product - if it receives a DNS request on a listener, it will respond to it.

The exception to this is if you have a listener configured on a floating IP address - in this case, only the BigIP which has that floating address will respond to a DNS request directed there.

Typically each GTM would have a separate listener. The listener is actually an LTM virtual server, and is not part of the GTM configuration, so it is not present in bigip_gtm.conf

GTM is simply a DNS resolver. The means by which requests are sent to the GTM is beyond the control of the GTM itself - that process is just the normal DNS resolution process whereby the NS records for your domain should point to your GTMs, and the resolver round robins the response.

In other words, if you have two GTMs, you should also have tow NS entries for the domains that your GTM resolves for. Those NS entries will be configured on the parent zone, and provide resolvers with the IP addresses of the name servers that resolve for that domain. This all happens before the GTM sees the request, so it can not influence this process.

In most cases, customers have two GTMs. Each has a listener configured in a subnet appropriate to the subnet it is connected to, and both listeners are configured as NS records. Requests are round robined by the normal DNS resolution process between both GTMs. Once either GTM in a sync group receives the query, the answer given will be consistent (ie, both GTMs would provide the same answer, regardless of which one is queried)