Erasing initial gains, the Sensex at the Bombay Stock Exchange was trading 11 points in the negative territory at 21,183 a short while ago.The 30-share index, which had gained 161 points the previous two sessions, rose over 100 points in early trade today, on sustained buying by funds and retail investors, mainly in metal, banking, consumer durables and realty sector stocks.The Nifty at the National Stock Exchange was also trading 11 points in the red at 6,303 a shortwhile ago.