Loopback Adapter

Posted on 2012-09-14
Last Modified: 2012-09-15
I overheard some Network engineers having a discussion about troubleshooting a problematic fiber port on a Cisco 3550 switch that had 24 - 10/100 ports and two 1Gbps ports using 1000Base-SX transceivers and fiber cables.   The one guy was describing that when troubleshooting a potentially bad fiber port that he would start by creating a loopback adapter on the interface.  

Can someone please explain what that means and how it would help to troubleshoot the fiber port?  This sounds like something that could be very useful if I understood him correctly.  

Question by:rotarypwr
    LVL 82

    Assisted Solution

    by:Dave Baldwin
    LVL 50

    Assisted Solution

    by:Don Johnston
    I think they meant "installing" a loopback adapter on the interface. Although, you could build your own which I suppose would qualify as "creating" it.

    Putting a known good loopback on a physical interface will allow the interface to come up without requiring a cable or a device at the other end. You will then know that the interface and transceiver is good.
    LVL 42

    Accepted Solution

    Most ethernet devices are autosensing meaning they determine which pair to use for transmit and recieve by detecting what is being transmitted from the other end of the connection.  Also integrated into the circuitry is the ability to have the port talk to itself which is known as a loopback.
    Whether a fiber connection has the same abilities will depend on the specifics of the hardware in question.
    Other loopback devices have been used on serial and parallel ports for decades and are the mating connector for the port with specific wires jumpered so the port talks to itself.
    But; there is a rub.  Loopbacks can test for grossly bad ports; they cannot test for weak ports or wrong voltage swings.  Many's the time I have seen serial, parallel and ethernet ports pass a loopback test only to have them fail when reconnected to the much longer wire (or fiber) with the real world loading at the other end.

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