AP Neighbor Messages (or lack of)

Neighbor messages, by default, are sent every 60 seconds over the air (DS bit set to 11) to multicast address 01:0B:85:00:00:00. They include the details like the list of neighbors that the AP hears, the RF Group name, channel it services clients on etc. This info is passed up to the WLC by the receiving AP.

This is what DCA & TPC are based on, so the absence of these neighbor messages is catastrophic to your RF environment.

Below is a picture of an AP's view of it's Rx neighbors (ap-beh-106-1), when these neighbor messages are not being sent:

The result of this is a higher than optimal Tx power and sub-optimal channel selection.

To watch these neighbor messages being sent, issue the following on the AP (you may also need to enable "terminal monitor"):

debug capwap rm measurements

You should see messages resembling the following. Take note of the "neighbor packet failed on <chan#>:

Neighbor interval timer expired, slot 0, band 0
*Nov 26 22:01:35.047: CAPWAP_RM: Triggering neighbor request on ch index: 2
*Nov 26 22:01:35.047: CAPWAP_RM: Sending neighbor packet #2 on channel 11 with power 1 slot 0
*Nov 26 22:01:35.047: CAPWAP_RM: Scheduling next neighbor request on ch index: 0
*Nov 26 22:01:35.047: CAPWAP_RM: RRM measurement completed. Request 2004, slot 0 status TIMEOUT
*Nov 26 22:01:36.835: CAPWAP_RM: Timer expiry
*Nov 26 22:01:36.835: CAPWAP_RM: Neighbor interval timer expired, slot 1, band 0
*Nov 26 22:01:36.835: CAPWAP_RM: Triggering neighbor request on ch index: 1
*Nov 26 22:01:36.835: CAPWAP_RM: Sending neighbor packet #1 on channel 40 with power 1 slot 1
*Nov 26 22:01:36.835: CAPWAP_RM: Scheduling next neighbor request on ch index: 2
*snip*
*Nov 26 22:01:56.903: CAPWAP_RM: Neighbor packet failed on 11

This shows the 2.4GHz radio failing to send neighbor packets. In this case it was on channel 11, but later in the debugs you see it for 1 & 6 as well. This was run on an AP neighboring the ap in the image above. This issue in particular is a result of a Cisco bug ( CSCul67156 ) and was affecting 2.4GHz only.

Below is a packet capture of [working] neighbor messages:

It is important to note these are sent out at lowest supported data rate and max Tx power. When looking at neighbor RSSI's, either in WCS/PI or "show ap auto-rf 802.11b <ap_name>" you'll see the RSSI of an AP at Tx level 1, even if RRM has chosen a level of 7/8. In other words, you can't use WCS/PI or CLI to gauge Co-channel. One option is WLCCA's 'compensated heard power' for that.

More reading on neighbor messages and RRM:
http://www.cisco.com/en/US/docs/wireless/controller/technotes/7.4/RRM_DG_74.html