Nimbostratus
1. GTM uses iQuery for intercommunication between GTM devices in the same sync group. This allows for communication about the health of other GTMs as well as configuration syncing. GTM uses what is known as a WideIP to direct traffic between datacenters. Anytime a record is served from a WideIP it is given a TTL of 30 seconds therefore it should only be cached for a maximum of 30 seconds. If that datacenter or standalone GTM were to fail, other GTMs will stop directing traffic to that WideIP/datacenter. Some users may see up to 30 seconds of failure before retrieving a new viable A record.
2. In order to direct traffic to your GTMs in various datacenters you'll want to create NS records for the various listeners on your GTMs. DNS is redundant by default in that if it cannot contact the first nameserver it attempts to query, it will automatically move on after a short delay. Therefore the ISP doesn't "select" which GTM, the client arbitrarily round robins the NS records.
3. See questions 1 and 2.
4. Connect an interface to each IP space and create a listener and NS record for both addresses.
GTM can seem a bit daunting at first, but once you get rolling it gets a lot easier. Soon enough you'll wonder how you ever lived without it. Feel free to post any other questions and I'll do my best to answer them. Hope this helps,
-George
