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Cisco#sh interfaces   GigabitEthernet0/0

GigabitEthernet0/0 is up, line protocol is up
  Hardware is CN Gigabit Ethernet, address is 6400.f13b.bec0 (bia 6400.f13b.bec0)
  Description: $ETH-LAN$$ETH-SW-LAUNCH$$INTF-INFO-GE 0/0$
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, **loopback not set**

Found 'loopback' statement under a port . Can anyone explain this why we require loopback under a port ?

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Every interface supports loopback for diagnostics. It's rarely used on an ethernet interface, but it's supported there as well. Loopback simply loops whatever it receives right back out -- usually without presenting it to the system.

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