How an AP joins a Wireless LAN Controller

In summary:

  • Boot
  • Discovery
  • Join
  • Code Current? No? Get new code and reboot
  • Get Configuration
  • Run

WLC Discovery

Methods of WLC discovery including:

  • Broadcast if WLC is on same VLAN as AP
  • Flash memory i.e. the AP remembers the last WLC it connected to
  • DHCP specifically option 43 – IP address of WLC
  • DNS looks for hostname CISCO-CAPWAP-CONTROLLER

Example configuration for Option 43 on a router:

ip dhcp pool vlan20
network 172.16.20.0 255.255.255.0
dns-server 172.16.20.1
default-router 172.16.20.1
option 43 hex f104.ac10.0a05

f104.ac10.0a05 is not a MAC address – it is a TLV (Type, Length Value)
f1 = data type
04 = 4 bytes of data
ac10.0a05 = 172.16.10.5 in hex.

Order of Precedence 

  • IP address of first WLC (if configured)
  • IP address of secondary WLC (if configured)
  • IP address of tertiary WLC (if configured)
  • IP address of master WLC (if configured)
  • Least busy WLC.

The Join Process

Two channels between the WLC and the AP.

  • Control Channel – DTLS (encryption) UDP 5245.  Responsible for code upgrades, configuration and heartbeats
  • Client Data – UDP 5247

 

 

 

 

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