Before-the-Bell: Redback Falls 24.5 Pct.  NEW YORK (Reuters) - Shares of Redback Networks Inc.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=RBAK.O target=/stocks/quickinfo/fullquote"&gt;RBAK.O&lt;/A&gt;, which makes networking equipment, fell 24.5 percent  in pre-market trading on Friday after it forecast third-quarter  revenue far short of Wall Street's estimates.