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Monitor traffic on modern network infrastructure
If you have managed switches in your network, and the devices of concern are attached to one them, you can use a setting to cause all traffic through a number of ports to be "mirrored" to one port. In effect you put one port on your switch into a sort of selective "reverse promiscuous" mode. Can't remember the commands/settings right now, been a while, and I usually rely on memory getting jogged once I am managing a switch . Check your switch documentation (ProCurve use a "mirror" setting option).