Troubleshooting Layer 1 Problems
The interface status codes, or interface state, play a key role in isolating the root cause of problems on serial links. In fact, the status on both ends of the link may differ, so it is important to examine the status on both ends of the link to help determine the problem.
For example, a serial link fails when just one of the two routers has administratively disabled its serial interface with the shutdown interface subcommand. When one router shuts down its serial interface, the other router sits in a down/down state (line status down, line protocol status down), assuming the second router is not also shutdown. The solution is to just configure a no shutdown interface configuration command on the interface.
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