When you configure GTM domain you also configure liveness test to perform validation on your each datacenter. Once the liveness test are configured, GTM replies with A or AAAA value of your live DC.
The liveness test are done by our system on internet. This systems are called as Web Agents. Each of your data centers is liveness-tested by several agents, chosen for proximity to your data center (closer is better) and for network diversity. Doing liveness test from multiple web agent reduces the possibility of an ISP wrong declaring your DC as down due to local network issue.
If all Web Agents are failing their liveness test, GTM marks all them up as GTM doesn't have any basis for preferring any of them. A data center is considered up if any of the servers in it are considered up.
When you configure liveness tests, GTM allocates a set of seven web agents for each of your data centers.
Normally, GTM attempts to allocate agents so that:
1. Agents are in the same continent as your data center
2. Agents are near your data center
3. No two agents are in the same ISP (network diversity)
4. Only live agents are assigned (sometimes agents are down for maintenance)
Sometimes it may be possible that you have a firewall in front of your web server and it may possible that your firewall may block GTM test agent. To permit, GTM test agent to performance liveness, please make sure your update your ACL list on firewall is updated to allow GTM Test Agent IPs. To help manage this, the portal has a Firewall Rules page that lists the IP addresses of all systems that might need to access your servers. For GTM, watch the entries that have Global Traffic Management in the Service column. If you have an ACL, ensure that all of the systems listed in the Firewall Rules page are entered into your ACL.
Please make sure you are subscribed to notification list so whenever there is an update to the list of web agent you can update your ACL list accordingly. GTM will not start using new web agent until its effective date has arrived. You will have about 3 weeks to update your ACL before GTM starts using new web agent for liveness test.