一台思科接入交换机3750V2，接入上层交换机也是3750V2，接入层交换机经常出现网络内所有电脑不通，管理vlan也连不上，使用console登录上去，查看到日志显示如下：
Nov 24 16:12:28.490 Beijing: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/1, changed state to down
Nov 24 16:12:29.496 Beijing: %LINK-3-UPDOWN: Interface FastEthernet1/0/1, changed state to down
Nov 24 16:12:32.843 Beijing: %LINK-3-UPDOWN: Interface FastEthernet1/0/1, changed state to up
Nov 24 16:12:33.850 Beijing: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/1, changed state to up
以上都是故障时间段重复出现的。并且show int fa1/0/1显示端口状态正常（上级和下级的互联端口都显示正常）：
FastEthernet1/0/1 is up, line protocol is up (connected)
而且上级的交换机show logging并没有显示端口的UPDOWN日志。
