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[SRX/AX] How to capture packets wirelessly when trying to troubleshoot association issues with the AX-411 access point

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Article ID: KB23178 KB Last Updated: 08 May 2012Version: 2.0
Summary:
This article provides information on how to capture packets at both the SRX and AX-411 level, to determine why packets are being dropped.
Symptoms:
Capturing packets at the AX411 access point and SRX.
Cause:
 
Solution:
To capture packets on the wireless radio and on the SRX, first you need to upgrade to at least Junos 11.1R1. Any previous versions will not be able to capture packets on the wireless radio.

To capture packes on the wireless radio and the SRX, perform the following procedure:

  1. Begin with the client being disassociated.

  2. Start the packet capture on the VLAN interface of the SRX device.

    root@% tcpdump -n -X -i vlan -s 512 > /var/tmp/srx_vlan_dump.pcap


  3. Open another SSH session to the SRX. With this second session, you can collect the pcap output at the VAP0 interface of the 2.4G radio and radio 2:

    root@> request wlan access-point packet-capture start MCP interface Radio2VAP0 duration 300 promiscuous file /var/tmp/vap0_dump.pcap

  4. Try to associate with an SSID from the client. After the client tries and fails to establish an IP, stop the capture:

    root@> request wlan access-point packet-capture stop MCP

  5. Collect the pcap dump on the AP's Ethernet interface:

    root@> request wlan access-point packet-capture start MCP interface Ethernet duration 300 promiscuous file /var/tmp/ap_eth_dump.pcap


  6. Try to associate with an SSID from the client. After the client tries and fails to establish an IP, stop the capture:

    root@> request wlan access-point packet-capture stop MCP


  7. Stop the capture on the SRX (from step 2) by pressing Ctrl-C.

The following files are created in the /var/tmp directory:

srx_vlan_dump.pcap - sniffer capture taken off the vlan interface on the SRX, connected to the AX-411
vap0_dump.pcap - sniffer capture taken of radio 2, virtual-access-point 0 (VAP0 interface)
ap_eth_dump.pcap - ethernet interace of the AX-411 interface

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