D-Link DGS-3212SR User Manual page 60

D-Link DGS-3212SR Layer 2 Gigabit Ethernet Switch
NOTE: You cannot mirror a fast port onto a slower port. For example, if you try
to mirror the traffic from a 100 Mbps port onto a 10 Mbps port, this can cause
throughput problems. The port you are copying frames from should always
support an equal or lower speed than the port to which you are sending the copies.
Also, the target port for the mirroring cannot be a member of a trunk group.
Please note a target port and a source port cannot be the same port.
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