It's called port mirroring. Some manufacturers also refer to this as SPAN ports but it's the same thing. On Cisco devices you issue the monitor session command twice to set this up. The first command sets up the source - which can be a VLAN or an individual port - and the second command sets up the destination (which has to be a specified port. Once you've done this you can plug your analyser into the destination port and watch all the traffic being handled by the source (you can specify inbound, outbound or both in the command). Note though that the destination port will not provide any other connectivity while the monitor session is in effect; all you can do on that port is sniff
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