equal load balancing in multi tier senario :

Thanks. So, this leaves perhaps two avenues of investigation;

1. A proxy is in use and is doing some sort of request multiplexing. A tcpdump on the external interface of the internal F5 might help you prove this. You should see initial requests from 'new' clients without the cookie being present, as it won't have been set yet. If you don't then there's a proxy doing something. Hope that makes sense.

    

2. Perhaps switch to the Round Robin load balancing method, this may at least prove something odd is going on with the connection counting when using Least Connections.

    

Also, I was wondering if you are using a session cookie or a particular time period before expiry?