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**
在端口下找到“环回”语句。谁能解释一下为什么我们需要在端口下环回?