GSLB - Monitoring LTM VIP load balancing via iRule

You should try configuring Prober Pools.

GSLB prober pools monitor health probes of the sync group servers to gather health and performance data about the sync group server resources.

All of the resources on your network are associated with a data center. BIG-IP DNS consolidates the paths and metrics data collected from the servers, virtual servers, and links in the data center. BIG-IP DNS uses that data to conduct load balancing and route client requests to the best-performing resource based on different factors. 

 DNS uses this data to optimize its load balancing decisions.

You can view the GSLB prober pools configured for the DNS sync groups you manage by going to Configuration > DNS > GSLB > Prober Pools From there, click a pool to edit it, or click Create to set up a new pool.

Benefits of F5 GSLB prober pools include:

• Faster app routing
• More efficient cloud scaling
• Optimized cloud migration
• Maximized app health, optimization, and protection 

BIG-IP^® DNS 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.

Prober selection and fallback options are available to provide a variety of monitoring choices for data centers and servers. Allowing the customization of prober selection for a particular environment may be useful in reducing false negatives due to network configuration issues and outages. The prober selection option specifies which BIG-IP system(s) are preferred when selecting a local prober. The prober fallback option controls what the BIG-IP system does when not enough probers of the type specified are available.