iRule to permanently assign www.abc.com/site1/ to webserver1 and www.abc.com/site2/ to webserver2

See if this helps:

when HTTP_REQUEST {
  if { [HTTP::uri] starts_with "/site1" } {
     node IPwebserver1 
  } elseif { [HTTP::uri] starts_with "/site2" } {
     node IPwebserver2 
  }  
}