> Does the GTM simply do a ping request to the LTM Virtual Servers every
> 30 seconds to check availability status
It's way more complex than that - the GTM schedules probe requests to big3d processes on LTMs in the Datacenter, which is then responsible for probing othet LTMs via iQuery. The status of virtual servers is determined directly from the LTM internal state tracking (based on the LTM pool monitors).
> and is the hold down timer 90 seconds before it's marked as Down?
This is controlled by the "ignore-down-response", which is disabled by default.
This will mark down LTM virtual servers immediately (because the defined pool monitor has already expired to mark the virtual server down).
If "ignore-down-response" is enabled, then the timeout period has to expire before the GTM marks the server object down.