关于cisco交换机端口自己down的解决办法
公司一个楼层交换机cisco2960的F0/21端口自己down了，一片怨声载道，人生鼎沸，妈了个B的。
通过命令：sh int f0/21 看到如下结果：
cisco12#sh int f0/21
FastEthernet0/21 is down, line protocol is down (err-disabled)
Hardware is Fast Ethernet, address is 0024.1351.2e95 (bia 0024.1351.2e95)
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, Auto-speed, media type is 10/100BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 19:29:25, output 19:29:20, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
130997686 packets input, [1**********]2 bytes, 0 no buffer
Received 3733082 broadcasts (0 multicasts) 0 runts, 0 giants, 0 throttles 38 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 0 watchdog, 3191174 multicast, 0 pause input
0 input packets with dribble condition detected
121355446 packets output, [1**********] bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out
通过红色部分了解端口处于故障状态，然后通过命令sh logging看看日志，结果如下： cisco12#sh logging
Syslog logging: enabled (0 messages dropped, 1 messages rate-limited, 0 flushes, 0 overruns, xml disabled, filtering disabled)
Console logging: level debugging, 219 messages logged, xml disabled,
filtering disabled
Monitor logging: level debugging, 0 messages logged, xml disabled,
filtering disabled
Buffer logging: level debugging, 219 messages logged, xml disabled,
filtering disabled
Exception Logging: size (4096 bytes)
Count and timestamp logging messages: disabled
File logging: disabled
Trap logging: level informational, 222 message lines logged
Log Buffer (4096 bytes):
hernet0/5, changed state to down
7w5d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
7w5d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
7w5d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
7w5d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
7w5d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
7w5d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
7w5d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
7w5d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
7w5d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
7w5d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
7w6d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
7w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
7w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
7w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
7w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
7w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
7w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
7w6d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
7w6d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
7w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
8w1d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
8w1d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
8w1d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
8w1d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
8w1d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed
state to down
8w1d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
8w4d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
8w4d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
8w4d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
8w4d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
8w5d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
8w5d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
9w1d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
9w1d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
9w3d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
9w3d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
9w3d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to down
9w3d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
9w4d: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
9w4d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up
9w5d: %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on FastEthernet0/21.
9w5d: %PM-4-ERR_DISABLE: loopback error detected on Fa0/21, putting Fa0/21 in err-disable state
9w5d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/21, changed state to down
9w5d: %LINK-3-UPDOWN: Interface FastEthernet0/21, changed state to down
9w6d: %LINK-3-UPDOWN: Interface FastEthernet0/19, changed state to up
9w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/19, changed state to up
9w6d: %SYS-5-CONFIG_I: Configured from console by vty0 (192.168.200.254)
9w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/19, changed state to down
9w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/19, changed state to up
红色部分知道了网络有loopback error detected，日吧。
找到问题了，解决吧。进入config模式下通过命令：errdisable recovery cause loopback， 心急了用show errdisable recovery命令看看还有多少秒完成，日，修复个错误还要几分钟。
至此，完成。我发誓再出现loopback error，我一定断他们的网，谁说都没用。