iRule to Select pool by parsing

Name the pools site1 and site2.... siteX..

when HTTP_REQUEST {  
   extract the first part of hostname.   
  set poolname [getfield [HTTP::header host] "." 1]   
  pool $poolname  
}  

To make sure you don't send user to an undefined pool name do this:

when HTTP_REQUEST {  
   extract the first part of hostname.  
  set poolname [getfield [HTTP::header host] "." 1]   
  if { [catch {pool $poolname} id ] } {  
      You can find this in the local traffic log from GUI or via : tail /var/log/ltm  
     log local0. "Pool name $poolname does not exist yet."    
   }  
}