GTM Design help

Here is one way to handle it:

 

 

Place a GTM at each of your datacenters. These two GTM's will be in the same sync group and will both be able to answer DNS requests. You don't necessarily have to make the GTM's authoritative for your entire domain if you do not want to. You can make it authoritative for individual records or subdomains, i.e:

 

 

www.domain.com IN NS gtm-dc1

 

www.domain.com IN NS gtm-dc2

 

 

Therefore, either GTM-DC1 or GTM-DC2 will answer DNS queries for www.domain.com. Because the GTM's are in sync (sync-group) they will both answer with the appropriate response (send client to DC1, or DC2 depending on your GTM configuration). The fact that you have one ISP with multiple external networks does not matter. The GTM will answer the DNS query based on logic that you configure within the GTM.

 

 

Your GTM's will be active/active in the sense that they will both answer DNS queries at all times, however GTM-DC2 can and will answer queries and direct users to DC1 if it needs to.

 

 

Hope this helps.