How to Create a RJ45 Ethernet Loopback Cable/Connector

Creating or making a RJ45 Ethernet loopback cable/adapter is pretty simple.

Ethernet Loopback Cable / Connector:

A loopback cable redirects the output back into input. This effectively gives the NIC the impression that it is communicating on a network, since its able to transmit and receive communications.

Hardware Required:

  • Crimper
  • Cat-5 cable (30cm should be enough)
  • Knife
  • RJ-45 connectors

Remove the cable pairs from the cat-5 cable and you need a single pair of approximately 4 inches length per loopback cable.

Next separate the 2 wires at each end of the cable enough to slide into to RJ-45 connector and follow below Pinout details.

Ethernet Loopback cable/connector  Pinout Details:

Pin 1 -> Pin 3

Pin 2 -> Pin 6

Next crimp the end cable using Crimping tool. Now RJ45 Ethernet Loop back Cable/Connector is ready.

RJ45 Ethernet Loopback cable

RJ45 Ethernet Loopback cable






Using the Ethernet loopback plug with Cisco Routers:







I covered how to use ethernet loopback cable for testing ethernet  or fastethernet interfaces on Cisco Routers or Swtiches.
Step 1   Do a “show interface” command on the suspect Cisco ethernet interface. If the interface is up and line protocol is down”

Step 2   Put a Ethernet loopback plug or connector on the suspect Cisco ethernet port. The show interface output should stay at an “up/down” status.

Step 3   In enable mode at the Router prompt enter the following (where o is the interface number):

Router#clear int fastethernet 0

Step 4   Check the interface status with show int Fastethernet 0 (where 0 is the interface number).

Step 5   You should now see the interface status become “up/up“, which indicates that the Cisco ethernet hardware is good. Otherwise hardware having issue.

Filed Under: Networking


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  1. Marina says:

    You have really interesting blog, keep up posting such informative posts!

  2. Robert says:

    Can any body tell me the color code to make the Loopback adapter jack for gigabit ethernet. I had tried on RJ-45 pin1 to pin3 & Pin2 to Pin 6 but it works with only 10/100 networking not in gigabit networking.

    • Kanth says:

      Did you try making an all 4-paired cross-over adapter?

      The same should work witih a cross-over patch cable (assuming all four pairs are crossed properly) if you plug each end into a switch.

      In both cases the Link light should be ON when plugged into a switch.

      I’m sure you’re thinking the right way, but what I’m saying is that given the scenario, cross all 4 pairs and it will work.

      The alternative is if you want a jack plug to just test if one port is working, and you want rx and tx on the Same Plug, then you need to jump:

      Pin 1 to Pin 3

      Pin 2 to Pin 6

      Pin 4 to Pin 7

      Pin 5 to Pin 8

      To make a jack test plug like this take your RJ45 plug and insert 4 wires into pins 1, 2, 4, and 5 and then loop them back in and mate with pins 3, 6, 7, and 8 consecutively. This plug adapter will work for 10/100/1000 ports.

      Hope that helps!

  3. Karla says:

    I am so much excited after reading your blog. Your blog is very much innovative and much helpful.

  4. Evan says:

    Loopback connectors won’t work with Gigabit ethernet, because there is an echo cancellor built into chip on nic that senses 100% crosstalk on pin 4,7, 5,8 if you can disable this function then it will work. I’ve been experimenting but can’t find setting’s to disable “NEXT”

  5. george says:

    Cool blog about RJ45 loopback cable,Thanks….

  6. Yash says:

    it was very useful for me.

  7. Muhammad says:

    I have tried this on LINUX +WINDOWS………both…..

    What I found is that…..The system will show u connected if NIC (Ethernet card) is/or set to 100Mbps (Fast Ethernet)……………on gig ethernets port will not be up…..either ur cable is cat6 or cat 5e………………….

    Good LUCK.

  8. Christian says:

    I was examining some of your content on this internet site and I conceive this internet site is very informative ! Keep posting .

  9. Jeremy says:

    I understand to make an RJ-45 connector into a hardware loop back connector. Thanks for post!

  10. Joshi says:

    Its excellent as your other content : D, regards for putting up.

  11. Chris says:

    great stuff mate for creating RJ45 loopback connector, thanks

  12. Chris says:

    great stuff i like the design

  13. Marianna Angelo says:

    Thanks for sharing. Great info.

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