LTM - Pool members active standby failover

Hi, 
First, you need to configure Priority Group activation: https://my.f5.com/manage/s/article/K13525153

This is a great feature when you want apply ( Active/Standby ) scenario on Pool level.

secondly: 

you need to enable Manual resume feature to prevent the traffic from being return to the down pool member when it become up again, this feature is configured on health monitor level , so I recommend to create a customized health monitor instead of the default one. 
Here the Manual Resume feature: https://my.f5.com/manage/s/article/K83316932

IF you don't want this solution however I recommend to proceed with my first approach , but there is an irule to do that , please review this Article on DevCenteral: Priority Group Activation ( Preemption question) | DevCentral

Thanks