GTM LB

1:1 ratio might as well be round robin. Keep in mind that at the global layer, the GTM is handing out resolutions to LDNS servers, so if you have 5 LDNS resolutions, 3 would go to site 1 and 2 would go to site 2. Now, if 90% of your clients happen to be servered by the 3 LDNS servers that are pinned to site 1 (until TTL expires or your persistence record expires), then you won't be at a 50% distribution at all, or even close. If you want even distribution across both datacenters, you're probably better off putting all the virtuals from both datacenters in a single pool and using the least connections, packet rate, or kbps load balancing mechanism.