Cisco AP Discovery and Join Process

In this lesson we’re going to take a look at the process required for our APs (access points) to discover and join WLCs (wireless LAN controllers).

3.0 Infrastructure

 3.3 Wireless

  3.3.c Describe access point discovery and join process (discovery algorithms, WLC selection process)

When an AP running in lightweight mode comes online, it it needs to connect to a WLC in order to function.

To achieve this, our AP completes two phases:

  1. Discovery Phase.

  2. Join Phase.

The first thing our lightweight AP will do when powered on is to try and discover as many controllers located within the network as possible.

Our AP will complete this straight out of the box, without the need for any configuration, also known as zero touch deployment.

Lightweight APs attempt to discover as many controllers as possible and create a list of available controllers to join.

The AP completes this in the following order.

1 – Layer 3 Broadcast

Our AP will start by sending a layer 3 broadcast packet on the local subnet our AP is located on.

This action is completed using a CAPWAP discovery message using UDP/5246.

Any wireless controllers on our subnet that receive our CAPWAP discovery message will respond with a unicast CAPWAP discovery response to our AP.

2 – Previously known WLCs

Cisco APs have the ability to remember up to 8 previously used controllers.

The AP will attempt to connect to these previously known controllers to see if their still active.

3 – DHCP Option 43

We have the ability to advertise the IP address of the controllers we want our APs to connect to with the use of a DHCP option. The IP address of the controller is provided to the AP via option 43.

4 – Resolve DNS hostname (CISCO-CAPWAP-CONTROLLER.localdomain)

Our APs then attempt to resolve CISCO-CAPWAP-CONTROLLER.localdomain via a DNS request.

If the AP learns a domain name via DHCP then this is used for the DNS request.
If we have the record configured within DNS, the AP will attempt to contact the WLC.

5 – Manually configured controllers

Our APs then attempt to resolve CISCO-CAPWAP-CONTROLLER.localdomain via a DNS request.

If the AP learns a domain name via DHCP then this is used for the DNS request.
If we have the record configured within DNS, the AP will attempt to contact the WLC.

Controllers that have been discovered during the discovery phase will report the following statistics to the AP:

If the AP is unable to discover any controllers as part of the discovery process then the AP reloads and begins the discovery process again.

It’s worth noting that our AP will use all methods in an attempt to discover as many controllers as possible, before proceeding to the join phase.

As long as our AP has discovered at least one controller to join as part of the discovery phase, our AP will then proceed with the join phase.

It’s possible that our AP has a list of controllers it’s discovered. Because of this, our AP will prefer controllers in the following order.

Step 1

Firstly, if our AP has joined a controller its discovered before – it will attempt to join this. If this is not the case, our AP has the ability to be configured with a primary, secondary and tertiary controller within its configuration. If this has been configured and a controller has been discovered that has been programmed as a controller in the list – it will attempt to join them in the specified order.

Step 2

Firstly, if our AP has joined a controller its discovered before – it will attempt to join this. If this is not the case, our AP has the ability to be configured with a primary, secondary and tertiary controller within its configuration. If this has been configured and a controller has been discovered that has been programmed as a controller in the list – it will attempt to join them in the specified order.