路由器接口NO SHUTDOWN 开启后,自动关闭怎么回事

Router>enable
Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface s0/1
Router(config-if)#ip address 13.1.1.2 255.255.255.0
Router(config-if)#no shutdown
Router(config-if)#
*Mar 1 00:03:26.671: %LINK-3-UPDOWN: Interface Serial0/1, changed state to up
*Mar 1 00:03:27.671: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1,
changed state to up
Router(config-if)#
*Mar 1 00:03:53.267: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1,
changed state to down