CCIE Enterprise Wireless (v1.0) – 3.9 Controller Mobility – C9800 Radio Resource Management – Part 2

How RF Groups are formed

  • When the WLC initializes as new, it creates a unique Group ID using the IP address of the WLC and a Priority Code.
  • The Priority Code is assigned based on the controller model and MAX license count (hardware limit) to create a hierarchical model and ensure that the controller with the most processing capacity is assigned the job of GL (Group Leader).
  • The Group ID and the RF Group Name will be used together in messages to other WLC’s and AP’s to identify them. Devices having the same RF Group Name will interoperate as members of the same RF Group.

The current controller hierarchy is as such:

8500 > 7500 > vWLC(large) > 5520 > 5760 > WiSM2 > 5508 > vWLC(small) > 3850 > 2500

When comparing Group IDs for leader election, the priority code is primary criteria and IP address is secondary. For instance, if there are 3 other controllers, none of which has the same or higher priority code than myself – I become the Group Leader. If all 3 have the same priority code as myself, then the one with the highest IP address wins and assumes the GL role.

For two WLCs to form an RF Group there is an infrastructure as well as OTA (Over The Air) component:

  • WLCs must be reachable to one another on the distribution network
  • They must each also have at least one AP that can hear the other’s NDP messages above -80 dBm

The distribution network communicates over unicast UDP:

Source Port Destination Port
RRM Manger 11b(11a) 12134(12135) 12124(12125)
RRM Client 11b(11a) 12124(12125) 12134(12135)

Neighbor Discovery Protocol or NDP, is sent from every AP/Radio/Channel every 60 seconds or less. The NDP packet is a special broadcast message that APs all listen for and it allows us to understand how every radio on every channel hears every other radio. It also gives us the actual RF path loss between APs.

Neighbor messages are sent to a special Multicast address of 01:0B:85:00:00:00, and are done so:

  • At the Highest Power allowed for the Channel/Band
  • The Lowest data rate supported in the band

For 802.11b this means that the message is sent at power level 1 (always the highest power for a particular radio) at 1 Mbps, and for 5 GHz radio’s 6 Mbps. This function is hard coded into the radio firmware, there is no user control. NDP power and modulation is not changed by user configured data rates or power levels.

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