step 1:

stop what you are doing, and label every SINGLE cable.

Every single one.

At both ends.

Power cable? label it.

Network cable? label it.

Fiber cable? label it, and indicate polarity of strands if you have separable connectors.

Serial cable? label it.

Then, once you are done labeling every single cable at both ends, document what is plugged into what.

Label every piece of gear with a unique label (unless you already have this).

Got a router? stick a unique number on it.

Got a docsis modem? stick a unique number on it.

Now, once you have every single cable labelled at both ends, and every single device with a unique number, now you can document what cable ID is plugged into what port of what device.

Draw yourself a simple little diagram of each device.

---------------------------------
| device:9                      |
|    [__] eth0                  |  eth0:cable11228
|                               |  eth1:cable11229
|    [__] eth1     [__] console |  console:cable33141
|                               |
---------------------------------

---------------------------------
|  device:13                    |
|    [__] gi1/1                 |  gi1/1:cable11229
|                               |  gi1/2:cable11228
|    [__] gi1/2    [__] console |  console:cable33142
|                               |
---------------------------------

Now, you have complete documentation of what plugs into what.

You can formulate statmenets like:

    device 9:eth0 connects to device 13:gi1/2 with cable 11228

    device 9:eth1 connects to device 13:gi1/1 with cable 11227

Now, you are ready.

Begin, by figuring out an appropriate length of cable for each cable number you have documented.

"Cable 11228 was 5 meters; it should be 7 feet. "

"Cable 11229 was 7 feet; length is good, but should be rerouted.

You needn't go through this whole list, but make note of how many of what lengths you should have.

Then, prepare your new replacement cable for cable 11228. Get a 7 foot cable, and apply a new label to it on both ends, with a new cable ID, say 200001.

add to your device-to-device documentation by adding your new cable ID:

    device 9:eth0 connects to device 13:gi1/2 with cable 11228 -> 200001

    device 9:eth1 connects to device 13:gi1/1 with cable 11227

Now, the day of the downtime.

arrange an outage window. shutdown everything. remove all cables. use your documentation to reconnect everything (find cable with 200001 at each end, plug it into device 9:eth0 and then into device 13:gi1/2).

repeat this process until everything is done, and you've routed everything correctly.

don't try to do a whole server room live. it's just a bad idea. if you have to, because you can't get an outage window, fine, but you will be able to make a much better result if you can install all your cables in one go instead of trying to "fix" the spider web 1 cable at a time.

source: https://www.reddit.com/r/sysadmin/comments/81y6eu/fixing_a_cabling_disaster/