Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter Guia Do Desenho
2-8
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 2 Cisco Unified Wireless Technology and Architecture
LWAPP Overview
WLC Discovery and Selection
The following section highlights the typical behavior of a Layer 3 LWAPP AP upon being reset. For a
comprehensive description of the discovery/join process, see the 440X Series Wireless LAN Controllers
Deployment Guide at the following URL:
comprehensive description of the discovery/join process, see the 440X Series Wireless LAN Controllers
Deployment Guide at the following URL:
Upon reset, the following sequence takes place:
Step 1
The AP broadcasts a Layer 3 LWAPP discovery message on the local IP subnet. Any WLC configured
for Layer 3 LWAPP mode that is connected to the same IP subnet will see the discovery message. Each
of the WLCs receiving the LWAPP discovery message will in turn reply with a unicast LWAPP discovery
response message to the AP.
for Layer 3 LWAPP mode that is connected to the same IP subnet will see the discovery message. Each
of the WLCs receiving the LWAPP discovery message will in turn reply with a unicast LWAPP discovery
response message to the AP.
Step 2
If a feature called ‘Over-the-Air Provisioning’ (OTAP) is enabled on a WLC, APs that are already joined
to that WLC will advertise their known WLCs in neighbor messages sent to other APs ‘over the air’. Any
new AP attempting to ‘discover’ WLCs for the first time will receive these messages and in turn unicast
an LWAPP discovery request to each WLC advertised in the OTAP message. (OTAP is not supported by
IOS APs in their initial state. In other words, a new IOS-based AP cannot use OTAP to discover a WLC.)
WLCs that receive LWAPP discovery request messages unicast an LWAPP discovery response to the AP.
to that WLC will advertise their known WLCs in neighbor messages sent to other APs ‘over the air’. Any
new AP attempting to ‘discover’ WLCs for the first time will receive these messages and in turn unicast
an LWAPP discovery request to each WLC advertised in the OTAP message. (OTAP is not supported by
IOS APs in their initial state. In other words, a new IOS-based AP cannot use OTAP to discover a WLC.)
WLCs that receive LWAPP discovery request messages unicast an LWAPP discovery response to the AP.
Step 3
The AP maintains previously learned WLC IP addresses locally in NVRAM. The AP sends a unicast
LWAPP discovery request to each of these WLC IP addresses. Any WLC receiving an LWAPP discovery
request responds by sending an LWAPP discovery response to the AP. As stated earlier, WLC IP
addresses can be learned via OTAP messages sent from existing APs already joined to WLCs. The
information stored in NVRAM also includes address information for any previously joined WLC that
was a member of another mobility group. (The mobility group concept is discussed in greater detail later
in this document.)
LWAPP discovery request to each of these WLC IP addresses. Any WLC receiving an LWAPP discovery
request responds by sending an LWAPP discovery response to the AP. As stated earlier, WLC IP
addresses can be learned via OTAP messages sent from existing APs already joined to WLCs. The
information stored in NVRAM also includes address information for any previously joined WLC that
was a member of another mobility group. (The mobility group concept is discussed in greater detail later
in this document.)
Step 4
If OTAP is not used, DHCP servers can be programmed to return WLC IP addresses using vendor
specific DHCP options. In this case “Option 43" is used in a “DHCP offer” to “advertise” WLC
addresses to LWAPP APs. When an AP receives its IP address via DHCP, it checks for WLC IP address
information in the Option 43 field of the DHCP 'offer'. The AP sends a unicast LWAPP discovery
message to each WLC listed in the DHCP Option 43. WLCs receiving the LWAPP discovery request
messages unicast an LWAPP discovery response to the AP.
specific DHCP options. In this case “Option 43" is used in a “DHCP offer” to “advertise” WLC
addresses to LWAPP APs. When an AP receives its IP address via DHCP, it checks for WLC IP address
information in the Option 43 field of the DHCP 'offer'. The AP sends a unicast LWAPP discovery
message to each WLC listed in the DHCP Option 43. WLCs receiving the LWAPP discovery request
messages unicast an LWAPP discovery response to the AP.
Step 5
In lieu of Option 43 information, the AP attempts to resolve the following DNS name:
“CISCO-LWAPP-CONTROLLER.localdomain”. If the AP is able to resolve this, it sends a unicast
LWAPP discovery message to each IP address returned in the DNS reply. As described above, each WLC
that receives an LWAPP discovery request message replies with a unicast LWAPP discovery response to
the AP.
“CISCO-LWAPP-CONTROLLER.localdomain”. If the AP is able to resolve this, it sends a unicast
LWAPP discovery message to each IP address returned in the DNS reply. As described above, each WLC
that receives an LWAPP discovery request message replies with a unicast LWAPP discovery response to
the AP.
Step 6
If after Steps 1 through 5 no LWAPP discovery response is received, the AP resets and restarts the search
algorithm.
algorithm.
Typically, either the DHCP or DNS discovery mechanism is used to provide one or more seed WLC
addresses, and then a subsequent WLC discovery response provides a full list of WLC mobility group
members.
addresses, and then a subsequent WLC discovery response provides a full list of WLC mobility group
members.
An LWAPP AP is normally configured with a list of up to 3 WLCs that represent preferred WLCs. If
these WLCs become unavailable or are over-subscribed, the AP chooses another WLC from the list of
WLCs learned in the discover response and chooses the least-loaded WLC.
these WLCs become unavailable or are over-subscribed, the AP chooses another WLC from the list of
WLCs learned in the discover response and chooses the least-loaded WLC.