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[WLA/WLC] How to collect wireless packet captures on 40 MHz channels in the 5 GHz spectrum via an AirPcap adapter



Article ID: KB24625 KB Last Updated: 05 Jun 2012Version: 1.0
This article provides information on how to collect wireless packet captures on 40 MHz channels in the 5 GHz spectrum via an AirPcap adapter. When troubleshooting a wireless system, you might need to collect wireless packet captures.
  • The goal of the wireless packet capture is to have an overview of the actual packets, which are being transmitted over the air, between the wireless client (laptop, PDA, smartphone, and so on) and the corresponding WLA.

  • With this 'packet choreography' being available, you will be able to analyze the behavior of either the client or the WLA and identify faults more accurately.

  1. Determine the test WLA:

    You will have to determine to which WLA, the test client is connecting. To do this, you will have to know the MAC address of the test client and type the following command at the controller CLI:
    # show sessions network mac-addr <mac_address> 

    The above output indicates that the test client is connected on WLA no.4, radio 2.

  2. Determine the operating channels of the test WLA:
    # show ap status

    As the above output, radio 2 from WLA no.4 is operating on channel 36+. The W flag indicates that the radio is operating on a bonded channel by using the upper channel; whereas the w flag indicates a lower channel is being used for bonding.

  3. Open Wireshark and select Capture > Options (or press Ctrl+K); the following window is displayed. Select the AirPcap adapter as the capturing interface and make sure that the Capture packets in promiscuous mode checkbox is selected:

  4. Now click the Wireless Settings button; the Advanced Wireless Settings window is displayed. You will have to select the primary channel (which is 36 in this example) from the Channel drop-down menu and the Channel Offset (which is +1, as the 40 MHz channel has been obtained by bonding with a upper channel):

  5. You are now ready to start the packet capture.

    Note: If an AirPcap adapter is not available, you can use a computer running the MAC OS. For more information, refer to KB24396 - [WLC] How to use built-in wireless capabilities of Mac OS X for capturing 802.11 traffic.

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