All traffic that goes through one or more LAN ports of the appliance can be duplicated into one designated mirror port. For example, all traffic that passes through LAN1 and LAN2 is duplicated into LAN5, which is configured as the mirror port. You can only configure one mirror port at a time.
Use Case – If an external device is connected to the mirror port, it receives all traffic that goes through LAN1/LAN2 of the appliance. This enables you to monitor traffic that goes through the appliance from the external device.
The mirror port is the opposite of the existing monitor port feature, in which the traffic from an external source such as a network switch or router goes into the (WAN) port of the appliance, so the appliance can inspect the traffic going through the external source.