AP joining process (1.b) -- What is Discovery mechanism - 4?

Hello everyone

In this post, I will explain that How to use Capwap discovery request and response to find out WLC  --- Discovery by DHCP. 

** Topology **
AP  ----   SW3750   ----   SWCat6K  ----   WLC5508

AP IP addr: 172.33.68.234
WLC5508 IP addr: 172.33.67.10

AP is in the Vlan 368 and WLC5508 is the Vlan 367

****  

Before to start, you need give the following commands on SW3750 and SWCat6K.

On SW3750, I use interface Gi3/0/4 to connect AP,
=====
interface GigabitEthernet3/0/4
 switchport access vlan 368
 switchport mode access
 spanning-tree portfast
====

I make dhcp pool on SWCat6k
=====
ip dhcp pool vlan368
   network 172.33.68.0 255.255.255.0
   default-router 172.33.68.254 

   option 43 hex f104.ac21.4310
=====

"option 43 hex f104.ac1f.fe37" is the point.  The option 43 is the hexadecimal string. Its sub-option is Type + Lenght + Value. Type is always the code 0xf1. Lenght is the number of controller management IP addresses times 4 in hex. Value is the ip address of the controller listed sequentially in hex.

For example in here, there is only one controller with WLC's management interface IP address is 172.33.67.10. then type is 0xf1. then length is 1 x 4 = 4 = 0x04. The IP address of wlc translates to ac.21.43.10(172.133.67.10) in hex.  So the command is  "option 43 hex f104.ac1f.fe37"

From the figure, you can see Discovery Type is DHCP(2) in Capwap discovery request. 

If you give "debug capwap client event" on AP,  you will see the similar messages. 

*Time: %CAPWAP-5-DHCP_OPTION_43: Controller address 172.33.67.10 obtained through DHCP

*Time: CAPWAP Control mesg Sent to 172.33.67.10, Port 5246

*Time: Msg Type   : CAPWAP_DISCOVERY_REQUEST

*Time: Msg Length : 99

*Time: Msg SeqNum : 0

*Time: %CAPWAP-3-EVENTLOG: Discovery Request sent to 172.33.67.10 with discovery type set to 2

*Time: CAPWAP Control mesg Recd from 172.33.67.10, Port 5246

*Time: HLEN 2,   Radio ID 0,    WBID 1

*Time: Msg Type   : CAPWAP_DISCOVERY_RESPONSE

*Time: Msg Length : 117

*Time: Msg SeqNum : 0

*Time: %CAPWAP-3-EVENTLOG: Discovery Response from 172.33.67.10

Our "What is Discovery mechanism" has finished. In next post, I will introduce some troubleshooting skill on "AP disjoins WLC""

See you next time and Good Luck!!

Reference 

1.

DHCP option 43 for lightweight cisco aironet access pints configuration

http://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/97066-dhcp-option-43-00.html