Cable testing on Juniper EX switches

22 09 2020

Does anyone use the cable test feature on EX switches? Did you even know you could do this kind of thing?

In case you didn’t, here is a bit of background and an example.

The Time Domain Reflectometer (TDR) test has been available on the EX switches for some time, but not a lot of people seem to know about it. What is TDR, I hear you ask?

A futuristic-sounding thing, what a TDR does is send a signal down a cable and measure how much (if any) is reflected by the far end. If the cable is damaged a little way along its length, the TDR test will tell you that distance to a fair level of accuracy.

In an ethernet cable, there are four pairs of conductors – eight wires in total. The operative ones are 1 & 2, and 3 & 6. The other conductors are unused but their presence in the Cat5 or Cat6 cable helps prevent cross-talk and signal attenuation. In a lab, usually cables are fairly short and easily replaced, but in a working environment, cables in floor-boxes under desks, or the structured wiring under the raised floor in an office often suffers quite badly from being crushed, bashed or otherwise assaulted.

For this reason, it is very useful to have a ready-made piece of test gear attached to every cable in your building ready to make a diagnosis. What better place for that test device to be than in your Juniper network switches?

Here’s how it works. First, you need to know what port you have an issue with. If the problem computer and the network switch are far apart, you can do that easily using ‘monitor start messages’ and getting someone to connect/disconnect the device so you can work out which port it is in.

Once you know which port it is, run the TDR test using request diagnostics tdr start interface <interface name>:

Then display the results using show diagnostics tdr interface <interface name>:

As you can see in the screenshot above, conductors 4 and 5 are ‘open’ meaning there is a cable break 1 metre from the switch port. If all conductors came up as ‘open’ this would mean the whole Cat5 cable was not plugged in, but since only a single pair out of the four is open, this means there’s something plugged in, but one pair is faulty. In this instance, tbe the break is 1 metre away, so we can be confident it is the patch lead from switchport to patch panel.

Here’s what it looks like when the cable has been removed from the switch port – all pairs are ‘open’ with a distance of 0 metres to the cable break:

Here is the same port with a new cable in at the wiring closet end, but nothing connected out at the desk port – 33 metres away as you can see:

Suppose you had some pairs open at 33 metres, and other pairs open at different distances? In that case, I’d say there’s nothing on the end of the cable, and that the structured wiring under the floor is damaged – mark the port on the patch panel as bad and avoid it!

TDR can tell you other things too – occasionally, this cable run was experiencing cross-talk too – strangely at 123 metres rather than the 31 metres we see above when there’s no device on the end. It is likely that the crosstalk messes up the TDR measurement somehow:

Here’s the same port, now with a device on the end of the cable run, and new patch cables installed – no open pairs at all now:

Now it is all looking good.

TDR can tell you a variety of other things – cable shorts, downshift to 10 or 100Mbps speeds where it should be 1G, swapped pairs etc. There is a more full description of the results of the test at the Juniper site here.


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