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Description:

Multiple WLAN controllers (WLC cluster) are often operated in networks to achieve redundancy. The access points are then distributed between the WLAN controllers to optimize the network load. If a WLAN controller fails, the access points connect to one of the remaining WLAN controllers.

This article describes how an access point selects the optimal WLAN controller in a WLC cluster scenario.



Discovering available WLAN controllers:

An access point broadcasts a discovery request to find all WLAN controllers available on the local network. Furthermore, a request is also sent to the DNS name wlc-address to be able to contact remote WLAN controllers.

This is done in the following circumstances:
  • An unconfigured access point is connected to the network.
  • The access point can no longer reach the currently used WLAN controller.
  • Every 30 to 60 minutes, an access point connected to a WLAN controller checks to see if a better WLAN controller is available.

The WLAN controllers respond with a discovery response, which the access point uses to create a list of the available WLAN controllers.



Criteria for selecting the optimal WLAN controller:

The selection of the optimal WLAN controller is based on the following criteria. The list is processed sequentially. The process ends when there is only one WLAN controller left to choose from.
  • Provision of an AP configuration: The access point evaluates the configuration that a WLAN controller can provide.
  • A WLAN controller with a specific access point profile takes the highest priority.
  • A WLAN controller with a default profile is given medium priority.
  • A WLAN controller without a profile is given the lowest priority.
  • The preference value: The access point calculates a preference value and assigns it to each WLAN controller. The value can lie between 0 and 255. The higher the value, the higher the WLAN controller is prioritized.

Note:
The preference value can only be assigned to a WLAN controller via the command line (Setup/WLAN-Management/Preference) or via WEBconfig (LCOS menu tree -> Setup -> WLAN Management -> Preference).
  • License usage: The access point checks the number of connected access points and the maximum possible number of access points. The lower the license usage, the higher the WLAN controller is prioritized.

    Note:
    If a WLAN controller has no further WLC licenses available for additional access points, it will no longer be considered for the current selection process.
  • CPU load: The access point receives information from the WLAN controller about its CPU utilization. The lower the CPU load, the higher the WLAN controller is prioritized.
  • Round trip time: The access point uses ICMP packets (ping) to check how fast the WLAN controller responds to requests. The lower the round trip time, the higher the WLAN controller is prioritized.



Automatic and manual checks for the ideal WLAN controller:
  • Access points automatically check the distribution at intervals of 30 to 60 minutes. If the WLAN controller used so far is included in the list of the remaining WLAN controllers, the access point will continue to use it. Otherwise, a random WLAN controller is selected.

    Note:
    If desired, this feature can be disabled on the WLAN controller using the following console command:

    set Setup/WLAN-Management/WLC-Cluster/WTP-WLC-Rediscovery no
  • In the case of manually initiated checks, the access points are randomly distributed to the WLAN controllers in order to use the license quotas as evenly as possible.

    Note:
    If desired, this feature can be executed on the WLAN controller using the following console command:

    do Setup/WLAN-Management/WLC-Cluster/Trigger-WLC-rediscovery-on-WTPs

    The manual check can also be initiated from LANmonitor. To do this, use LANmonitor to connect to one of the WLAN controllers and switch to the WLAN controller tree. Right-click on Active APs and click on the context-menu entry Start discover WLC on APs.