Port Mirroring

Port mirroring is an approach to monitoring network traffic that involves forwarding a copy of each packet from one network switch port to another.
Port mirroring enables the administrator to keep close track of switch performance by placing a protocol analyzer on the port that's receiving the mirrored data.
An administrator configures port mirroring by assigning a port from which to copy all packets and another port to which those packets will be sent. A packet bound for -- or heading away from -- the first port will be forwarded to the second port as well. The administrator must then place a protocol analyzer on the port that's receiving the mirrored data to monitor each segment separately.
Network administrators can use port mirroring as a diagnostic or debugging tool. 

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