It happens to everyone. You thought you had a stash of zip ties (cable ties, tie wraps, whatever) in your tool bag and you just found out your all out. Sometimes you have to secure cables in a plenum environment but you don't have the plenum rated zip ties. The barrel knot will fill in nicely and you can use wire or string to tie it.
The barrel knot is the old school zip tie. This was taught to me by a Marine who had learned it by reading a WWII era field manual on cabling.
Step 1: Choose the Material for the Knot
You want to use something strong with a relatively small diameter. The wire inside a cat5e or cat6 cable will work great. You'll want to straighten it out as much as possible.
Here I am using 24 gauge cross connect wire. You can buy a roll of it here
Step 2: Prep the Wire
Straighten out the wire as much as possible. Then make a "u" with the wire. The length of the "u" will determine the length of the knot.
Step 3: Begin Wrapping the Cables
Lay the "U" parallel with your cables and begin wrapping the extra wire as neatly as possible around the cables. You will want to keep this as tight (for strength) and as neat (so you can take pride in your work) as possible.
Step 4: Insert the Wire Through the Loop
Now insert the wire through the loop.
Step 5: Pull the Loop Through
Now pull the loop under the wraps. At this point your knot is as good as done. But if you want to go pro. Follow the next step.