Explanation/Reference:
Section: Considerations for Expanding an Existing Network Explanation
Explanation:
When you add a lightweight access point (LAP) to a wireless network that uses Lightweight Access Point Protocol (LWAPP), the LAP goes through a sequence of steps to discover and register with a wireless LAN controller (WLC) on the network. Because a new LAP has not been configured with a static IP address, the LAP will first attempt to obtain an address from a Dynamic Host Configuration Protocol (DHCP) server.
When the LAP receives an IP address, the LAP scans the DHCP server response for option 43, which identifies the address of a WLC. Although this method is always the first action taken by a new LAP when it attempts to discover a WLC, the LAP will also use other methods.
When the LAP receives an IP address from the DHCP server, the LAP can also receive other configuration parameters, such as the IP address of a Domain Name System (DNS) server. If a DNS server is configured, the LAP will attempt to resolve the host name CISCO-LWAPP-CONTROLLER.localdomain, where localdomain is the fully qualified domain name (FQDN) in use. Once the LAP has resolved the name to one or more IP addresses, the LAP will send an LWAPP discovery message to all of the IP addresses simultaneously.
Alternatively, a LAP can use Over-the-Air-Provisioning (OTAP) to discover a WLC. OTAP is enabled by default on a new LAP. With OTAP, LAPs periodically transmit neighbor messages that contain the IP address of a WLC. A new LAP that has OTAP enabled can scan the wireless network for neighbor messages until the LAP locates the IP address of a local WLC. Once the LAP has discovered the IP address of a WLC, the LAP will send a Layer 3 LWAPP discovery request directly to the WLC.
If Layer 2 LWAPP mode is supported, a new LAP can attempt to locate a WLC by broadcasting a Layer 2 LWAPP discovery request message. If there are no WLCs on that network segment or if a WLC does not respond to the Layer 2 broadcast, the LAP will then broadcast a Layer 3 LWAPP discovery request message.
A new LAP will not have the address of a WLC stored in nonvolatile random access memory (NVRAM) by default. However, you can configure a LAP with the IP address of a WLC to facilitate the discovery of a WLC when the LAP is installed. In addition, if a LAP has ever joined with a WLC, it may store the previously discovered WLC IP address as a primary, secondary, or tertiary WLC.
Reference:
Cisco: Lightweight AP (LAP) Registration to a Wireless LAN Controller (WLC): Register the LAP with the WLC