[ixp1200] Transmit Problem

Hi, I need to design an Intrusion detection system on IXP1200, When I tried to filter the packets based on some rules, I used the datastream (simulation) that matches with one of the rules and hence all the packets received(on port 0) should be dropped. When I debug I was able to see that the control never goes to buffer_enqueue(), and always the buffer_free() is being called, but still when I observed the log file( on port 1) I could see some packets being transmitted, How is this possible? Anybody got this kind of problem before? Pl respond. thanx, malathi