Intrusion, Inc., makes a tap with a single output: If the sum of the streams collected from the two inputs exceeds the capacity of the single output, packets are dropped. “Real” taps like the Finisar UTP IL/1 below or the TopLayer Fast Ethernet Copper Tap have two inputs and two outputs: With two outputs, how do you recombine the streams? Calvin Gorriaran told me OpenBSD’s pf can be used to bridge the two interfaces listening for tap inputs.”]
Source: https://taosecurity.blogspot.com/2003/07/while-perusing-focus-ids-mailing-list.html