NDP packets on Cisco 3700 abnormally low

This post is a follow up from last weeks one-way audio issue. Although we did resolve the one-way audio by disabling A-MSDU we also found a bug on the Wireless LAN Controller that caused Neighbour Discovery Protocol (NDP) packets to be abnormally low and thus causing the TX power of the Access Point (AP) to be excessively high (power level set to 1) and channel assignment not working at all. I’ve noticed this bug twice in the last month at different customers.

This bug only affected the 5GHz radios as seen in the diagrams below and not 2.4GHz. Diagram 1 shows power levels for 5GHz on the right and Diagram 2 shows power levels for 2.4GHz on the right for the same AP’s.

5GHz - Power level 1

2.4GHz - Power level 8

I monitored ten AP’s in a particular area. Seven of these AP’s would stay on power level 1 and would not change while the other three AP’s in this area would change its power level on the 10 minute mark (as per default) and change between power levels 2-5 on a continuous basis. On the second occasion I saw this the client had about 25 AP’s on site and all of them was stuck on channel 36 and power level 1.

This could be the root cause of one-way audio. It is important to understand that wireless communication occurs in a bi-directional manner. Uplink communication from the client to the AP is not always the same as downlink communication from the AP to the client. While an AP will send beacons downlink to the VoWLAN handset, most surveying tools will only display information as it pertains to downlink transmissions; therefore some problems are not easily detected using pre- or post-site survey tools. While a post site survey is vital after deploying the WLAN, a survey tool may not take into consideration the uplink signal being transmitted by the  wireless IP phone in comparison to the downlink signal.

Most access points will often have a higher transmit power + the antenna gain than the VoWLAN handset. When this occurs, the IP phone will still hear the downlink frames sourced from the AP, but the AP will not hear the uplink frames from the wireless IP phone and this leads to possible one-way audio.

The workaround prescribed by Cisco would be to manually assign channel/power settings which could turn into a timeous process or as the update of this bug shows from 6 November 2014 you can upgrade to the releases mentioned below.

The equipment used in this case are:

  • Cisco 5508 Wireless LAN Controller
  • Cisco 3700 series Access Point

The software affected by this bug (CSCuq86750) are:

  • 7.6(120.0)
  • 8.0(100.0)
  • 8.0(102.9)

The known fixed releases are:

  • 7.6(130.10)
  • 7.6(130.204)

Reference      http://www.cisco.com/c/en/us/td/docs/wireless/technology/vowlan/troubleshooting/vowlan_troubleshoot/2_Gen_Troubleshooting_Guidelines.html#wp1054631  https://tools.cisco.com/bugsearch/bug/CSCuq86750

 

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NDP packets on Cisco 3700 abnormally low

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