ERSPAN is the acronym for Encapsulated Remote Switched Port Analyzer. It mirrors traffic on one or more source ports and delivers the mirrored traffic to one or more destination ports. The traffic is encapsulated in Generic Routing Encapsulation (GRE), which is therefore routable across a Layer 3 network between the source switch and the destination. In this case, the destination is the IP of the monitor interface (e.g. ‘mon1’) on the Plixer FlowPro appliance.