If you configure a port as a monitor port for miiroring, why does the command
sh int gi slot /port ,show that
Gigabit ethernet is down, line protocol is up?.
It's the mechanism to prevent the port on participating on any of the Layer2 protocols (STP, CDP, VTP, etc). It also disables ingress traffic on the port so the device attached to it, can't send traffic - just receive the SPAN traffic.