Switching - show cable-diagnostics

The show cable-diagnostics command found on Cisco Catalyst switches is a diagnostic tool that can display information about the physical cable connected to a particular interface.

The command has two forms:

show cable-diagnostics tdr show cable-diagnostics prbs

The first uses what is known as Time Domain Reflectometry (TDR), which is a methodology of determining the characteristics of copper wiring by sending electrical waveforms on the wiring and observing the waveforms that are reflected back.

The second uses what is known as PseudoRandom Binary Sequence (PRBS), which is method of generating statistically random bits on the wire. The command will cause the port to become a PRBS generator. The generated bits, which appear as an electrical waveform on the wire, can be used to determine the state of the cable by examining the reflected waveforms.

The tdr option is only supported on FastEthernet and GigabitEthernet ports, while the prbs option is only supported on 10GigabitEthernet ports.

The output of both commands is similar and includes information about the length of each pair of copper wires, as well as their statuses.