How to Deploy GTM for DR site failover

Setup two GTM's internet facing, one at each site.

Configure a subdomain to delegate to the GTM. Change your application's DNS record to a CNAME of a new wideip address on your GTM. Create a pool with both application virtual servers, set the order so the normal server is always first by using Global availability. Assign that pool to your wideip.

DNS delegation
wip.mycompany.com in NS gtm1.mycompany.com.
wip.mucompany.com in NS gtm2.mycompany.com.
gtm1.mycompany.com in A X.X.X.X
gtm2.mycompany.com in A Y.Y.Y.Y

DNS mapping
myapp.mycompany.com in CNAME myapp.wip.mycompany.com.

WideIP Configuration
myapp.wip.mycompany.com
     pool myapppool
         virtual myapp.mycompany.com
         virtual dr_myapp.mycompany.com

Server Configuration
Server primary_myapp.mycompany.com X.X.X.X
    Data Centre Primary
    Virtual myapp.mycompany.com X.X.X.X

Server dr_myapp.mycompany.com Y.Y.Y.Y
    Data Centre DR
    Virtual dr_myapp.mycompany.com Y.Y.Y.Y

Data Centre Configuration
    Name Primary
    Name DR

You will need to add some health checks suitable for your environment. See askf5.com GTM monitors for details. If your application are hosted on BIGIP LTM's the GTM's can talk to them to discover the virtual's so you do not need to add them manually.

When you make a request to myapp.mycompany.com the DNS will pickup the domain delegation and ask GTM to resolve the name. GTM will check if your primary site is up and return the address of that application server X.X.X.X. If the application server at that site is down it will return the DR site Y.Y.Y.Y