Active-Active Data Center

High level pre-reqs are simple

1. Both DC's available from you client networks
2. LTM and GTM
3. Whoever is designing and implementing this must know how your apps work (It never goes well if this step is skipped, but sadly I find it often is)

That doesn't tell you a great deal though. And I'm not sure how much detail you need or what exactly you're stuck on.

1. What GTM needs is to be authoritative for whatever domain your'e using (I favour a subdomain like gtm.your.domain.com which has your application FQDN in it. And use a CNAME in your service domain).
2. Secondly your GTM needs to be able to talk to your LTM's using the 3dns service. Your LTM's then report the status of the VS's up to GTM so GTM can use that information to decide where to resolve the queries to.
3. Decide what algorithms you're going to use to dispatch at the GTM level.
4. Implement it and test

Note that GTM doesn't perform redirects. It performs DNS resolution. Your client asks their local DNS to resolve a name (service.domain.com). Using CNAMES as above your normal DNS resolves that to service.gtm.domain.com which is queried from the GTM server. GTM then looks at the request and matches it to a wideIP and the wide runs it via the chosen algorithms and out pops an IP (One of the VS's associated with the wideIP). The IP for that VS is then returned to the client. The client opens a TCP connection and (eventually) makes the query (HTTP/whatever)

H