Cisco’s Flexible Radio Assignment (FRA)
I have heard about Cisco FRA for a while but I am only starting to see this out in the field. This technology offers great advancements over statically assigned Radios.
There are two modes of operation in FRA Macro/Macro cell and Macro/Micro cell. I will only be discussing the Macro/Micro mode in this blog. The Macro/Micro cell will have a large cell and a smaller cell inside which will increase capacity on your 5 GHz network.
The theory behind FRA is if you design a network for 5 GHz then you will more than likely have too much 2.4 GHz coverage. This is why FRA is only run against the 2.4 GHz radios.
There are only two AP models that work with FRA. They are the 2800/3800. When the AP creates a Micro cell, the power will always be set to the minimum power of the AP. In the case of the 3802, this would be 2 dBm.
How it Works
FRA uses the Neighbor Discovery Protocol (NDP) from RRM to figure out if there is too much coverage on the 2.4 GHz band. The output of this calculation is called Coverage Overlap Factor (COF). You can set the threshold for the COF at Low 100%, Medium 95% and High 90%. When FRA sees too much coverage based on these thresholds values, it will mark the radio as redundant. Once it is marked redundant it can be assigned another role. There are three states (roles) these radios can be in 2.4GHz/5GHz/Monitor Mode. Depending on the COF the controller will either leave it at 2.4GHz, change it to 5 GHz or put it in Monitor Mode. When the controller puts an AP in Monitor mode the only way to fix this is to reset the AP.
The AP can suppress Probe responses from one of the radios. When the APs receives Probe requests on both the Macro and Micro cells within a short period of time from a client who is not associated, the AP can suppress the Probe Responses on the radio which it doesn’t want the device to join. When a client is associated to either radio on the AP, the AP will suppress the Probe Response from the other radio. This should help prevent the client from roaming between radios. The Probe Suppression option is disabled by default on the controller.
FRA will monitor the cells and keep devices that are similar on the same radio. This will help improve throughput. FRA will use 802.11v, 802.k and Probe Suppression to keep the same type of clients on the same radio.
Pros and Cons of FRA
- FRA will give you more capacity in the 5 GHz band.
- FRA eliminates of fixes the balance between 5 GHz and 2.4 GHz radios on your wireless network.
- The controller will limit how many devices can be on the Micro cell.
- If your device authenticates to the Micro Cell and moves away from the Micro cell area. This could force it to roam to the Macro cell, which would increase roaming. These additional roaming events force the device to stay awake more which will affect battery life. Cisco has safeguards against this but just like RRM, it doesn’t always work.
- If you have 2.4 GHz clients your network, the coverage area after FRA runs could change dramatically. Depending on how often you have FRA run, this can lead to a less stable network. I know Devin Akin (@DevinAkin) would say that 2.4 GHz is dead and probably should be at this point especially for voice clients, but I just did a job last week where they insisted using 2.4 GHz for voice.