Cisco APs ignore DHCP Option 43 and use CAPWAP UDP broadcasts



Cisco Access Points (APs) are a key component of many enterprise networks, providing access to local and wide area networks for mobile devices. As such, it is important to have them configured correctly to ensure maximum reliability and performance. One important configuration setting is the use of DHCP Option 43, which can be used to direct the APs to a specific controller or Access Point Manager (APM). However, in some cases, Cisco APs may ignore DHCP Option 43 and instead use CAPWAP UDP broadcasts to locate the controller.

The CAPWAP protocol is used by Cisco devices to discover and manage WLAN controllers and access points. It uses UDP broadcasts to send messages between controllers and access points, which allows devices to locate a controller without requiring manual configuration. This is useful in environments where the controller is not always located at a known IP address, or where the IP address of the controller may change due to DHCP leasing or other reasons.

When an AP is first powered up, it will send out a CAPWAP discovery broadcast to locate a controller. If it does not receive a response, it will then attempt to use DHCP Option 43 to locate a controller. If the Option 43 is configured properly, the AP will receive the address of the controller in the DHCP response.

However, in some cases, the AP may ignore DHCP Option 43 and continue to send CAPWAP discovery broadcasts. This can happen if the DHCP server is not configured correctly, or if there is an issue with the Option 43 configuration. In these cases, the AP will not receive the controller address in the DHCP response, and will continue to send CAPWAP discovery broadcasts until it finds a controller.

Fortunately, this issue can be resolved by properly configuring the DHCP server and ensuring that the Option 43 is configured correctly. The Option 43 should include the IP address of the controller, and should be configured as an array of bytes. Additionally, the DHCP server should be configured to provide the Option 43 in the DHCP response. Once these steps have been completed, the AP should be able to locate the controller using DHCP Option 43, and should no longer be sending CAPWAP discovery broadcasts.

In summary, Cisco APs may ignore DHCP Option 43 and use CAPWAP UDP broadcasts to locate the controller. This can cause issues with AP management and reliability, and is typically caused by an incorrect configuration of the DHCP server and Option 43. To resolve this issue, the DHCP server and Option 43 should be configured properly, and the AP should be able to locate the controller using DHCP Option 43.

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