Only heavy forwarders can route or filter data at the event level. Universal forwarders and light forwarders do not have the ability to inspect individual events except in the case of file header extractions, but they can still forward data based on a data stream's host, source, or source type. They can also route based on the data's input stanza, as described below, in the subtopic, "Route inputs to specific indexers based on the data's input".
http://docs.splunk.com/Documentation/Splunk/6.0/Forwarding/Routeandfilterdatad
