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Very often, configuration and
troubleshooting on routers is done remotely when it is not possible to
physically inspect the router connections. The show
controllers serial command is
useful to determine the type of cable connected without inspecting the
cables. 
By examining the
show controllers serial
command output, the type of cable that the controller detects can be
determined. This is useful for finding a serial interface with no
cable, the wrong type of cable, or a defective cable.
The
show controllers serial
command queries the integrated circuit (chip) that controls the serial
interfaces and displays information about the physical interface. This
output varies from controller chip to controller chip. Even
within a router type, different controller chips may be used.
Regardless of the controller type, the
show controllers serial
command produces a tremendous amount of output. Other than the cable
type, most of this output is internal technical detail regarding the
controller chip status. Without specific knowledge of the integrated
circuit, this information is of little use.
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Lab Activity
e-Lab Activity: Troubleshooting Using show
controllers serial
In this lab, the students will use the
show controllers
command to see what cable is attached to the serial interface.

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