GTM Pool communication

When you have multiple GTMs, only one of them will be responsible for choosing who will monitor. That could be one of the GTMs, or any other BIG-IP device. GTM will first try to pick a BIG-IP in the same data center.

In your case is possible that is picking, for example, an LTM in that data center, if you have one. Also, telnet does not do the same test as the monitor, in case you are using something more complex than a TCP monitor.

Enable this db key, and see if you can get more information:

tmsh modify sys db gtm.debugprobelogging value enable

This will log to /var/log/gtm. If in production, don't leave that enable and enable just during the tests.

You can also use prober pools, to tell who you want to monitor.