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Troubleshooting Serial Lines
385
Notice the seventh line of the output. It says that the counters were cleared
28 minutes before. After the counters are cleared, you can associate any
new data with current network events. If you try to associate current net-
work events with inaccurate data, you will never find the problem.
Now let's go through the available data provided by the
show interface
serial
command. Refer to the output previously listed.
The first line provides information regarding the status of the interface
and the line protocol:
Serial1 is up, line protocol is up
In this case, both are up and functional. If the interface is down, the line
protocol must also be down.
Cabling problems, carrier problems, or hardware problems can all be
reasons for a serial interface to report as down. These problems can
be addressed by verifying proper cable connectivity, replacing hardware
(including cables), and checking the CSU/DSU for carrier signal. If
you cannot resolve the problem by using these techniques, you can and
should contact the local carrier, who can verify the carrier service.
Another possibility for the interface status is that the interface is up, but
the line protocol is down. When this happens, it can be a variety of problems,
as follows:
Failed CSU/DSU
Router interface problems
Mismatched timing on CSU/DSU or carrier network
Misconfigured interface
Keepalive signals not received from remote router
Carrier problem
You should verify that the local interface and the remote interface are
properly configured. Loopback tests can be performed. These tests will be
discussed in the CSU/DSU section of the chapter.
Continuing with the description of the output of the
show interface
serial
command, you see that the second line of the output displays the
hardware type of the interface:
Hardware is HD64570
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