Packet reset when call Virtual Server

I can simulate the case, but I cannot find the root case and cannot find the solution. Can anyone help? Below is my environment load balancing design

 

 

Basically, I've 4 servers, let say A, B, C & D assigned to Virtual Server. But B only will backup A when A is down and D only will backup C when Server C is down instead of simple round robbin load balance to all 4 server.

 

 

So, I've created Pool-1 include Server A & B, but A priority group is higher than B. And I also create Pool-2 include Server C & D, and C priority group will higher than D.

 

 

But only Pool-3 included all A, B, C & D servers assigned to my Virtual Server. Because of B only backup A and D only backup C. But I need apply an iRule (sample below) to handle B backup A only and D backup C design.

 

 

LB_Failed {

 

If A down;

 

perform reselect Pool-1;

 

elseif B down;

 

perform reselect Pool-2;

 

elseif C down;

 

perform reselect Pool-1;

 

elseif D down;

 

perform reselect Pool-2;

 

}

 

 

All the above config. is suggested by F5 and well tested on another environment. However, many packet reset found suddently. But this error disappeared after I used a simple round robbin load balance traffic to all 4 servers or the caller directly make request to the server real IP address. Therefore, the problem should not because of physically connection problem.

 

 

Many thanks