Apach Server Service health check

Sir

 

I have 2 apache web servers by bigip v9 to do load balance,and I set http health check,if I manual to disable one apache web server,and in health check period(5 seconds/15 seconds interval),if this time client access this web server,client can't brower this web page,and I find bigip also direct to service down web server,can't direct to another normal web server,bez when bigip hasn't detect one node service is down,it also direct traffic to this,now if I try to use iRule to do,how to do?

 

 

Thank for your support!!