故障描述
经用户反馈，一台cisco2960x接入交换机的一个端口插网线不通，ip电话也没有poe供电。
排查过程
查看交换机端口状态，发现变成了errdisable：
ZH_HQN_SW2960X_04#show int g1/0/14
GigabitEthernet1/0/14 is down, line protocol is down (err-disabled)
从官方文档找到恢复的方法：
errdisable recovery cause all
接下来就可以查看errdisable的详细信息了：
ZH_HQN_SW2960X_04#show errdisable recovery
ErrDisable Reason Timer Status
----------------- --------------
arp-inspection Enabled
bpduguard Enabled
channel-misconfig (STP) Enabled
dhcp-rate-limit Enabled
dtp-flap Enabled
gbic-invalid Enabled
inline-power Enabled
link-flap Enabled
mac-limit Enabled
loopback Enabled
pagp-flap Enabled
port-mode-failure Enabled
pppoe-ia-rate-limit Enabled
psecure-violation Enabled
security-violation Enabled
sfp-config-mismatch Enabled
small-frame Enabled
storm-control Enabled
udld Enabled
vmps Enabled
psp Enabled
dual-active-recovery Disabled
evc-lite input mapping fa Disabled
Recovery command: "clear Disabled
Timer interval: 300 seconds
Interfaces that will be enabled at the next timeout:
Interface Errdisable reason Time left(sec)
--------- ----------------- --------------
Gi1/0/14 loopback 293
可以看到，所有的errdisable恢复类别都被激活，并且底部显示了处于errdisable状态的端口，报错的原因，以及还剩多久自动恢复(time left)。
继续查找报错原因loopback，官方给的解释如下：
Loopback error：A loopback error occurs when the keepalive packet is looped back to the port
that sent the keepalive. The switch sends keepalives out all the interfaces by default. A device
can loop the packets back to the source interface, which usually occurs because there is a
logical loop in the network that the spanning tree has not blocked. The source interface
receives the keepalive packet that it sent out, and the switch disables the interface
(errdisable).
意思是交换机默认会从所有端口发出keepalive报文，如果发出keepalive的源端口接收到自己发送的keepalive报文，那么交换机就会认为产生了环路(这个环路没有被生成树阻塞)。
交换机会把这个端口设置为errdisable状态。
确认端口恢复后，记得关闭errdisable recovery恢复功能：
no errdisable recovery cause all