We have two GTM-Virtual in sync group in two different data centers
If we disable the group sync, all virtual servers will be marked UP
But if we establish sync between them, they'll be marked down, and the pool members will show error :
Offline (Enabled) - Monitor /Common/bigip from /Common/GTM_SH : no reply from big3d: timed out
Which means it is trying to monitor the VS through the other GTM machine but not succeeding , which is weird why it is even trying to monitor the VS through the connected GTM machine and not locally as it was succeeding in doing so before establishing the sync connection.
The workaround is really easy. Just configure a prober pool that has the GTM(DNS) that haves access to the F5 LTM devices:
BIG-IP Global Traffic Manager (GTM) performs intelligent probing of your network resources to determine whether the resources are up or down. In some circumstances, for example, if your network contains firewalls, you might want to set up device-specific probing to specify which BIG-IP systems probe specific servers for health and performance data.
Also you may enable debug to better see which DNS device marks the VS as down:
Did you enable debug to see in /var/log/gtm which GTM/DNS system is usually selected for monitoring when the Sync group enabled? If the prober pool members are used is a good question.
I suggest also uploading qkviews and checking the bug tracker https://support.f5.com/csp/bug-tracker for ''big3d: timed out'' as I found https://cdn.f5.com/product/bugtracker/ID1108237.html .
Also big3d should be the same on all devices as this can cause issues and all GTM should have run the gtm_add script to be part of a sync group.
K13312: Overview of the BIG-IP DNS big3d_install, bigip_add, and gtm_add utilities (11.x - 16.x)
K13734: BIG-IP DNS synchronization group requirements
If you get stuck maybe go to support.