Cisco CCNA Exam Tutorial: Troubleshooting Directly Connected Serial Interfaces
CCNA exam success depends largely on noticing the details, and this is especially true of configurations involving directly connected serial interfaces. And of course, it’s not enough to notice these details – you’ve got to know what to do about them!
A Cisco router is a DTE by default, but directly connecting two DTEs with a DCE/DTE cable is not enough. In the following example, R1 and R3 are directly connected at their Serial1 interfaces. The line goes up briefly after being opened, but the line protocol goes down after about 30 seconds.
R3(config-if)#int s1
R3(config-if)#ip address 172.12.13.3 255.255.255.0
R3(config-if)#no shutdown
2d18h: %LINK-3-UPDOWN: Interface Serial1, changed state to up
2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up
R3(config-if)#
2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down
The problem is that one of the routers needs to act as the DCE in order for the line protocol to come up and stay up. If this were...
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