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WL0005 - WLC Access Point Registration (Part 1)

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The video walks you through different methods that Cisco Access Point uses to discover Wireless LAN Controller and how an AP makes a decision on which controllers to join. Each method will be demonstrated with their order of priority explained. We will mention the purpose of Master Controller and stress on importance of controller priming. Wireshark analysis will be performed throughout the lab for us to examine controller discovery and AP registration processes. 
Part 1 of this video covers the basic of controller discovery with L3 and directed L3 broadcast
  • Controller Discovery
  • L3 Broadcast
  • Directed L3 Broadcast
  • DHCP Option 43
  • DNS Resolution
  • Master Controller
  • Controller Priming
  • Controller Failover

About Author

Metha Cheiwanichakorn, CCIE#23585 (RS, Sec, SP), is a Cisco networking enthusiast with years of experience in the industry. He is currently working as a consulting engineer for a Cisco partner. As a founder of and an instructor at, Metha enjoys learning and challenges himself with new Cisco technologies.


1) Why it resolving DNS domain name after got the IP address from the DHCP ?
2) After resolving the Domain i mean DNS , Again why it is sending the DHCP request ?
3) At the time of Ap was connected to Vlan 64, the AP broadcasted the At the time
it doent know any thing about the controller . then second time how the Ap will send the request
directly to controller.

1-2) AP goes through all discovery method and build a list of candidate WLC. If multiples are found, it picks one from method with highest priotity.

3) Once AP joined WLC, AP will remember and try to contact the same WLC at next boot up.

What is the last process name
ip help-protocal udp 5246 in the switch. may i know process name.

It's called UDP forwarding. It takes a broadcast packet for the specified port/protocol and turns it into unicast to forwarded address. By default, only DHCP and some other protocol are forwarded. For CAPWAP, you need to explicitly add it.

What is the main difference between the L3 broadcast and Direct L3 Broadcast.

Broadcast normally stays within L2 domain. Directed broadcast allows you to send broadcast packet in a remote subnet by using the subnet bradcast IP.


my cisco ap 1702i is stuck at traslating "CISCO-CAPWAP-CONTROLLER"..... domain server ( and would not join my controller Cisco 4402. kindly note this is a lab environment and no DNS server. i have switch, wlc and two APS. Thanks

Please first check the 4402 even supports 1700 APs, and if so that you are running correct code. Assuming all thing supported, AP should be able to find WLC when on same subnet.