Cisco Access Points and Selecting a Wireless LAN Controller

After the discovery process of a lightweight access point; it should of built a list of live wireless LAN controllers that it can communicate with.

The lightweight access point begins a separate process of selecting a single wireless LAN controller and sending a CAPWAP Join Request to it. The access point will wait until the wireless LAN controller responds with a CAPWAP Join Response.

Once the CAPWAP Join Response message has been received by the access point, it will build a Datagram Transport Layer Security (DTLS) tunnel to secure any further CAPWAP control messages.

There are three steps to the wireless LAN controller selection process:

• If the access point has previously joined a wireless LAN controller and has been preconfigured with first, second, and third preferences to wireless LAN controllers it will try join them in that order.
• If the access point does not know of any of the candidate wireless LAN controllers, it will try discover one. If the controller has been configured as a master controller for that access point, it will respond to the access points request.
• The access point will attempt to join the least-loaded wireless LAN controller to help load balance access points across a set of controllers. Each controller will respond with its load during the discovery phase. The load is defined as the ratio of number of already joined access points to the maximum number of access points supported.

If the access point gets rejected by a wireless LAN controller when trying to join it, there may be a few reasons:

• The controller has reached the maximum number of access points it can support

In a situation where the number of access points trying to join a wireless LAN controller exceeds that of what the licence or capability of the wireless LAN controller, a priority system can be introduced to access points. All access points have a default priority of low but can be set with low, medium, high, or critical.

If an access point with higher priority tries to join the controller that is full, a lower priority access point will be kicked off the network.