Cisco 2800/3800 APs support Flexible Radio Assignment, which allows the 2.4 GHz radio to flip to either a monitor or another client-serving 5 GHz radio. When the AP is operating in this dual-5 GHz mode, the normal radio (slot 1) powers the macro cell, and the flexible radio (slot 0) powers the micro cell. The terms macro and micro are used for two reasons. When the flexible radio is put into 5 GHz mode, either automatically through the FRA algorithm or manually, the radio switches to a more directional antenna than the normal 2.4 GHz antenna. See the antenna radiation patterns of the 3800 from the AP2800/3800 Deployment Guide:
|AP3800 Antenna Patterns|
Looking at the elevation pattern (right), you can see that the macro radio has a "dead spot" directly below the AP. The micro radio (blue line) has a 15 dB advantage over the macro radio in this dead spot. Unfortunately because of the power limit on the micro radio, most clients will still perceive the macro radio as having a higher signal strength. This is even more true when not directly under the AP.
In order to take advantage of the micro cell, the AP/Controller has to have a way to nudge clients that connect to the macro cell over to the micro cell. This mechanism is called client steering, and the default method to steer clients is the 802.11v BSS Transition Request.
To see how client steering works with flexible radios the following settings must be made on the controller:
- Flexible Radio Assignment must be enabled globally under Wireless -> Advanced.
- The flexible radio in the AP2800/3800 can either be set to Auto or client serving. When in auto mode, the FRA algorithms determines if it is better to leave the radio in 2.4 GHz client serving mode, monitor mode, or 5 GHz client serving mode. For my testing I manually configured the flexible radio as 5 GHz client serving.
- BSS Transition Management must be enabled for the WLAN that the clients will connect to.
It's also helpful to see the BSSID values that are assigned to the macro and micro cells, to be able to confirm values in the BSS Transition Management requests.
Note the difference in the last octet of the BSSIDs between the macro and micro cells. I will reference this later.
The command to list the client steering parameters, their default values and explanation are shown below:
I connected the first client to the SSID and confirmed through the CLI that it had connected to the macro cell. When using an AP setup in macro/micro, you will see extra lines in the 'debug client' output that contain XOR:
The first client is not eligible for transition to the micro cell because it is the only client in the macro cell. Let's see what happens when the second client connects to the macro cell.
We see in the second line that the debug output shows a RSSI value of 212. I'm not sure how this scale of RSSI equates to a power level in dBm, but it appears to be above the threshold of -55 dBm. The third line indicates that the client will be scheduled for transition.
Before the client is transitioned, it sends a 802.11k Neighbor Report request. The debug output is interesting.
Because the client-steering engine had already decided to transition this client to the micro cell, it limits the list of neighbors it will send back to the BSSIDs on the micro cell.
Note that the transition of the client is scheduled; it doesn't happen immediately. Perhaps this is to prevent flapping of clients transitioning between the micro and macro cells as clients join and leave the cell. It may also be delayed to allow clients in motion to roam to other macro cells, instead of being pulled back into the micro cell of a far away AP. My testing indicates that the amount of time that elapses between the association of the client that triggers the XOR roam and the transmission of the 802.11v BSS Transition Management Request varies. If I find more information I will update this blog.
Below we see the sequence of events as the second client is transitioned to the micro cell.
Line 1 is the debug entry for sending the 11v BSS Transition Request, which is acknowledged in line 4. The client re-associates in line 5. Line 6 indicates that the client is associating to the XOR radio slot 0, which is the micro cell. In line seven we see that the client is not eligible for transition from the micro back to the macro cell: the difference in the number of clients between the cells (0) is not greater than the transition client balancing window (1).
Over the air, we see a BSS Transition Management request that includes a candidate list. The only entry in the candidate list is the BSSID of the WLAN on the micro radio.
|BSS Transition Management Request|
|BSS Transition Management Response|
The important fields here are the BSS Transition Response Status Code and the BSS Transition Target BSS. The Status Code communicates whether the client accepts or rejects the request, and a value of 0 indicates that client accepts it. The Transition Target BSS indicates the BSSID that the client intends to transition to. In this case, it matches the BSSID in the candidate list from the request frame.
Now we see that the second client has been transitioned to the micro radio on slot 0.
That's it for Part 1 of this series on client steering between macro/micro cells on an AP3800. Next I will look at what can be done if clients do not support 11v.