Saturday, August 26, 2017

FRA and Macro/Micro Cell Operation - Part 2

Part 1 of this blog series looked at how Cisco 2800/3800 APs running in dual-5 GHz mode can steer clients from the macro cell to the micro cell using 802.11v BSS Transition Management frames. In this installment, I will look at what methods can be used if your clients don't support 802.11v.

Before going into the details of the other method (probe suppression), here is what I have observed while testing a mix of clients:
  • Both Android and iOS devices responded well to 802.11v Transition Management Requests. Sometimes the iOS device I was testing with would reject the request with a reason code 6, but most of the time it accepted the transition. 
  • If there are enough clients connected to the macro cell to warrant a transition to the micro cell, a client that does support 802.11v will be moved, even if it was in the macro cell "first." 
  • According to the latest RRM White Paper, if a client does not support 802.11v, but does support 802.11k, it can be transitioned, but not as gracefully. The client must request a neighbor report, and the returned neighbor list will be limited to the BSSID of the micro cell. The client will then be disassociated, after which it will hopefully connect to the micro cell. I was not able to replicate this; it was hard to find a client that supported 11k but not 11v. Turning off 802.11v on the WLAN resulted in no clients being transitioned at all, whether or not they supported 11k. 
Configuring probe suppression is shown below. Probe suppression can be configured to suppress only probe responses, or both probe responses and auth responses. 

(Cisco Controller) >config advanced client-steering probe-suppression enable probe-and-auth

(Cisco Controller) >show advanced client-steering

Client Steering Configuration Information

  Macro to micro transition threshold............ -55 dBm
  micro to Macro transition threshold............ -65 dBm
  micro-Macro transition minimum client count.... 1
  micro-Macro transition client balancing win.... 1
  Probe suppression mode......................... probe-and-auth
  Probe suppression validity window.............. 100 s
  Probe suppression aggregate window............. 200 ms
  Probe suppression transition aggressiveness.... 3
  Probe suppression hysteresis................... -6 dBm

The macro to micro transition threshold has a similar meaning with probe suppression as it did with 11v transition. If a new client is a transition candidate, probes received on the macro radio with an RSSI stronger than the macro to micro threshold will have their responses suppressed.

Probe suppression steering introduces four new parameters, only two of which are user configurable. The parameters perform the same function as those under Wireless -> Advanced -> Band Select, but have slightly different names.

The probe suppression aggregate window is an amount of time that a burst of probes from a client on a single change are considered a single probe. This is similar to the Scan Cycle Period Threshold value in Band Select. Sometimes clients will sends out probes in bursts of multiple probes. Below is a Motorola G4 probing out on 5 GHz. It sends bursts of 5 probes on the same channel, just milliseconds apart. The client-steering engine will treat these 5 probes as a single probe because they all happened within 200ms.

Probe Bursts

The probe suppression validity window is the amount of time that could elapse between probes (or bursts of probes) from a single client received on the macro radio. The default value is 100 seconds, and it acts as an age-out timer.

The validity window works with the transition aggressiveness value, which corresponds to the probe cycle count value under Band Select. The transition aggressiveness value sets a limit on the number of times probe responses from the macro radio will be suppressed. The default is 3. If a probing client was a candidate to have probe responses from the macro cell suppressed, and the client had probed out on the macro channel 3 times within 100 seconds, the fourth probe (or burst) on the macro radio would be answered. This allows clients to connect to the macro cell if they refuse to connect to the micro cell because the RSSI at the client is too low.

The probe suppression hysteresis is a user configurable value between -3 and -6 dBm, with the default being -6. When Cisco uses the word hysteresis, it refers to a dampening method to prevent clients from bouncing back and forth between radios. In the context of Client Roaming, under Wireless -> 802.11a/b, the hysteresis value tells CCX clients to move to a new AP only if the RSSI value is 3 dB better than the current AP. I stumbled across the meaning of the hysteresis in probe suppression by trying to adjust the values of the transition RSSI thresholds.

(Cisco Controller) >config advanced client-steering transition-threshold macro-to-micro -60

Value must be greater than micro to Macro RSSI - probe suppression hysteresis

(Cisco Controller) >config advanced client-steering transition-threshold micro-to-macro -60

Value must be less than Macro to micro RSSI + probe suppression hysteresis

In this case, it looks like the -6 dBm hysteresis means that probes for clients already associated to the AP would have to be 6 dB weaker/stronger to get moved to the other cell. This makes sense, as you don't wont the client bouncing back and forth between the micro and macro cells because of small differences in RSSI that could just be from different client device orientations.

My testing with probe suppression for client steering was mostly subjective. Since the clients did not associate, I could not use "show client detail" to see the RSSI of the probe requests at the AP. I could definitely see probe suppression in action over the air. Below is a capture on channels 44 and 161. The macro cell was on channel 161, and you can see probes on 161 being ignored.

Probe Suppression of Macro Cell
The client connects to the micro cell on channel 44.

Other testing I conducted involved the transition aggressiveness factor. My Moto G4 cycles through the 5 GHz channels in about 6 seconds. With a transition aggressiveness factor of three, it should take about 24 seconds to see probe responses from the macro cell. My observations lined up with this prediction within a few seconds.

Overall, I didn't find the probe suppression method of client steering to be as predictable as the 11v method, but it did work satisfactorily. Given that most clients now support 11v I would prefer using that method over probe suppression.

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