How to Register the LAP with the WLC?

The WLCs and Cisco LAPs are part of the Cisco Unified Wireless Network architecture. The Cisco Unified Wireless Network architecture centralizes WLAN configuration and control on the WLC. The LAPs cannot act independently of the WLC. The WLC manages the LAP configurations and firmware. The LAPs are "zero touch" deployed, and no individual configuration of LAPs is required.
In order for the WLC to be able to manage the LAP, the LAP should discover the controller and register with the WLC. After the LAP has registered to the WLC, LWAPP messages are exchanged and the AP initiates a firmware download from the WLC (if there is a version mismatch between the AP and WLC). If the AP's onboard firmware is not the same as the WLC's, the AP will download firmware to stay in sync with the WLC. The firmware download mechanism utilizes LWAPP. Then, the WLC provisions the LAP with the configurations that are specific to the WLANs so that the LAP can accept client associations.
These WLAN-specific configurations include:

• Service set identifier (SSID)


• Security parameters


• IEEE 802.11 parameters, such as:
  
  
  • Data rate
  
  
  • Radio channels
  
  
  • Power levels
  
  

There are different methods that a LAP uses in order to discover the WLC. This document discusses the different methods that the LAP can use in order to register the WLC. But first, the document explains the sequence of events that occur when a LAP registers with the WLC.
Note: The Management interface is the default interface for in-band management of the WLC and connectivity to enterprise services such as AAA servers.The management interface is also used for layer two communications between the WLC and access points. The Management interface is the only consistently "pingable" in-band interface IP address on the WLC.
Note: A WLC has one or more AP Manager Interfaces that are used for all Layer 3 communications between the WLC and the lightweight access points after the access point discovers the controller. The AP Manager IP address is used as the tunnel source for LWAPP packets from the WLC to the access point, and as the destination for LWAPP packets from the access point to the WLC. The AP Manager must have a unique IP address. Usually this is configured on the same subnet as the Management interface, but this is not necessarily a requirement. An AP Manager IP address is not pingable from outside the WLC. Refer to the Configuring Ports and Interfaces section of Wireless LAN Controller Configuration Guide for more information.

How to Register the LAP with the WLC?
This sequence of events must occur in order for a LAP to register to a WLC:

1. The LAPs issue a DHCP discovery request to get an IP address, unless it has previously had a static IP address configured.


2. The LAP sends LWAPP discovery request messages to the WLCs.


3. Any WLC that receives the LWAPP discovery request responds with an LWAPP discovery response message.


4. From the LWAPP discovery responses that the LAP receives, the LAP selects a WLC to join.


5. The LAP then sends an LWAPP join request to the WLC and expects an LWAPP join response.


6. The WLC validates the LAP and then sends an LWAPP join response to the LAP.


7. The LAP validates the WLC, which completes the discovery and join process. The LWAPP join process includes mutual authentication and encryption key derivation, which is used to secure the join process and future LWAPP control messages.


8. The LAP registers with the controller.

The first problem that the LAP faces is how to determine where to send the LWAPP discovery requests (step 2). The LAP uses a hunting procedure and a discovery algorithm in order to determine the list of WLCs to which the LAP can send the discovery request messages.
This procedure describes the hunting process:

1. The LAP issues a DHCP request to a DHCP server in order to get an IP address, unless an assignment was made previously with a static IP address.


2. If Layer 2 LWAPP mode is supported on the LAP, the LAP broadcasts an LWAPP discovery message in a Layer 2 LWAPP frame. Any WLC that is connected to the network and that is configured for Layer 2 LWAPP mode responds with a Layer 2 discovery response. If the LAP does not support Layer 2 mode, or if the WLC or the LAP fails to receive an LWAPP discovery response to the Layer 2 LWAPP discovery message broadcast, the LAP proceeds to step 3.


3. If step 1 fails, or if the LAP or the WLC does not support Layer 2 LWAPP mode, the LAP attempts a Layer 3 LWAPP WLC discovery.

See the Layer 3 LWAPP WLC Discovery Algorithm section of this document.

4. If step 3 fails, the LAP resets and returns to step 1.

Note: If you want to specify an IP address for an access point instead of having one assigned automatically by a DHCP server, you can use the controller GUI or CLI to configure a static IP address for the access point. Refer to the Configuring a Static IP Address on a Lightweight Access Point section of the WLC Configuration guide for more information. If the LAP is assigned a static IP address and cannot reach the WLC, it falls back to DHCP.

…More detailed examples of Layer 2 & Layer 3 LWAPP WLC Discovery Algorithm and AP Fail-over Between Different Mobility Groups you can refer to http://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/70333-lap-registration.html

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