Big3d Timeout on F5

  After the F5 GTM upgrade to 14.1.4.6 it is observed that all the VIP on the GTM are flapping. We observed that this was matching multiple errors. Here the VIP will show a log saying it turned green to red because of big3d timeout. 

Basically, this is a timeout from GTM to LTM

Below were the articles that we saw matching our situation where we thought it is the problem is the resources on the LTM. Since it is pegging up we increased the resources on the LTM.

https://my.f5.com/manage/s/article/K35326235

We tried the recommended action as in below.  But this did not work. 

Recommended Actions

One or more actions can be taken on the impacted BIG-IP device to free up resources so that big3d can respond in a timely manner:

1. Add additional CPU cores to VM or vCMP guest
2. Reduce the size and/or complexity of the BIG-IP configuration

If the above actions do not work or are not feasible, then you may increase the BIG-IP monitor timeout value on BIG-IP DNS:

1. Create a new BIG-IP monitor and increase the probe timeout from 90 seconds (default) to a new value.  Increase in 5 second increments and test to see if the timeout messages stop.
2. Select the new BIG-IP monitor for the impacted BIG-IP server Health Monitor.
3. Click Update.

Timer Change :

Then we tried changing the timer. We created a monitor as in the below settings and configured it to all GTM. What i did not do is we have to apply the same monitor to all the LTM on the server. 

Further, there was a recommendation to update the timers. F5 recommends that the timeout value should be equal to 3 times the frequency ( interval) +1 . Example is below. Also they recommend that the interval value should be shorter. Below is the optimum settings that they suggested.