Promiscuous Mode / tcpdump not seeing traffic

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http://forums.fedoraforum.org – Have an installation where I have setup a secondary quad NIC that we want to use as a network monitoring card. We ran ifconfig and set eth3 in Promisc mode and when you run ifconfig it successfully shows the NIC in that mode, however, when we run tcpdump, we get nothing (sitting on a SPAN port on the switch). If we unplug from the switch and plug in a Windows box with Wireshark and monitor that port, we successfully get all the traffic which means it has got to be a problem on the linux box. Where can I find any more info on what I might be missing to successfully setup this interface to monitor traffic. (HowTos)