谁知道sdhl的线路怎么总是启不来

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我在两端的cisco 2620的串口上配了地址，用sdhl连接。\r\n电信说线路正常，但我的路由不断显示以下信息：\r\n07:38:38: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up\r\n07:38:58: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to down\r\n07:39:08: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up\r\n07:39:28: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to down\r\n07:39:38: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up\r\n07:39:58: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to down\r\n\r\n各位看看是怎么回事？\r\n\r\n谢谢