Cisco Cisco 2000 Series Wireless LAN Controller Guia De Resolução De Problemas

Cisco recommends that you have knowledge of these topics:

Basic knowledge of the configuration of LAPs and Cisco WLCs

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Basic knowledge of Lightweight Access Point Protocol (LWAPP)

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Refer to Cisco Technical Tips Conventions for more information on document conventions.

In a Cisco Unified Wireless network, the LAPs must first discover and join a WLC before they can service
wireless clients.

Originally, the controllers only operated in Layer 2 mode. In Layer 2 mode, the LAPs are required to be on
the same subnet as the management interface and the Layer 3 mode AP-manager interface is not present on
the controller. The LAPs communicate with the controller using Layer 2 encapsulation only (ethernet
encapsulation) and do not Dynamic Host Configuration Protocol (DHCP) an IP address.

When Layer 3 mode on the controller was developed, a new Layer 3 interface called AP-manager was
introduced. In Layer 3 mode, the LAPs would DHCP an IP address first and then send their discovery request
to the management interface using IP addresses (Layer 3). This allowed the LAPs to be on a different subnet
than the management interface of the controller. Layer 3 mode is the dominate mode today. Some controllers
and LAPs can only perform Layer 3 mode.

However, this presented a new problem: how did the LAPs find the management IP address of the controller
when it was on a different subnet?

In Layer 2 mode, they were required to be on the same subnet. In Layer 3 mode, the controller and LAP are
essentially playing hide and seek in the network. If you do not tell the LAP where the controller is via DHCP
option 43, DNS resolution of "Cisco-lwapp-controller@local_domain", or statically configure it, the LAP
does not know where in the network to find the management interface of the controller.

In addition to these methods, the LAP does automatically look on the local subnet for controllers with a
255.255.255.255 local broadcast. Also, the LAP remembers the management IP address of any controller it
joins across reboots. Therefore, if you put the LAP first on the local subnet of the management interface, it
will find the controller's management interface and remember the address. This is called priming. This does
not help find the controller if you replace a LAP later on. Therefore, Cisco recommends using the DHCP
option 43 or DNS methods.

When the LAPs discover the controller, they do not know if the controller is in Layer 2 mode or Layer 3
mode. Therefore, the LAPs always connect to the management interface address of the controller first with a
discovery request. The controller then tells the LAP which mode it is in the discovery reply. If the controller is
in Layer 3 mode, the discovery reply contains the Layer 3 AP-manager IP address so the LAP can send a join
request to the AP-manager interface next.