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Cisco Networking: Data Link Layer Troubleshooting

In the Open System Interconnection (OSI) model, the data link layer is just above the physical layer. At this layer, you should check the following items:

  • Link status: Most operating systems provide some sort of tools to check on the link status. Microsoft Windows always seems to tell you when you have connected or disconnected a network card. Make sure the network card is enabled in the operating system, if it is, then move on to the next steps.

  • Check the status lights: Most network cards have one or two lights. These lights will identify link status and speed, as well as activity over the link. What information is displayed is based on the manufacturer settings, so you may need to check your documentation if they are not labeled. If you look at the switch your computer is connected to, you will also see status lights.

    These lights will show you link status, speed, duplex settings, and activity. If you only have lights on one side of the connection, then you likely have a physical issue, like a break in some of the wires inside of the cable. If you have connection lights that agree at either end of the connection, such as both reporting gigabit connection speed, then you are partway to verifying data link connectivity.

    If there are no status lights, then check your physical connections again.

  • Attempt using Address Resolution Protocol (ARP): Test to see if other computers on the network are visible at all. ARP is a command line tool that will display the MAC addresses of all devices with which you have recently communicated.

    This means that you can attempt to communicate with another device, and then check your ARP cache. If you can see other computers, then your connection at the data link layer is likely working correctly.

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