The Bombay Stock Exchange benchmark Sensex on Monday opened 171 points higher on buying in banking, auto and metal stocks amid firming Asian trends. The 30-share index gained 171points, or 0.83 per cent, to trade at 18,039 points. The index had lost 123 points in the previous session. The wide-based National Stock Exchange's Nifty also regained 5,400 points by rising 51.90 points, or 0.96 per cent, to 5,419 points. The Hong Kong's Hang Seng index was up by 1.37 per cent, while Japanese Nikkei by 0.47 per cent in morning trade.