XS Apps security question

Friends

we have a requirement in our project. For which, we did not get much help.

suppose there is a firewall between the internet, extranet (DMZ) and intranet.

We currently have BIG IP in the landscape. The XS team suggests to use the web dispatcher, however, my understanding of the web dispatcher is that it does a round robin to the available nodes. My understanding of big ip is that it is a true load balancer which will redirect the request to the available server in the cluster.

1. Can we set up the web dispatcher to use a specific server or set of servers instead of going directly to the master ? from previous experience, the master is usually busy working on other requests, therefore we want to use a true balancer such as big ip to redirect the requests to the slaves (1+ slaves depending on our environments)
2. If using BIG IP, can we still achieve the same results as the Web Dispatcher ?

   3. Any suggestions on the architecture of extranet application security is greatly appreciated

Thanks in advance for your suggestions