Controller Based WLANs

How does LMS preemption (LMS failback) work?

by on ‎07-02-2014 08:31 AM

Product and Software: This article applies to all Aruba controllers and ArubaOS 3.2 and later.

 

One of the AP redundancy feature is to configure LMS and backup LMS under AP system profile so that if the LMS goes down, the AP will bootstrap to the backup LMS. However, with ArubaOS 3.1 code, the AP will not failback to the primary LMS automatically. The user must force the AP failback by rebooting the AP. This is to avoid unnecessary AP bootstrap in case of network instability. However, sometimes this is problematic for the administrator.   

 

Starting with ArubaOS 3.2, an LMS failback feature has been introduced. With this feature, the AP will failover back to the primary LMS when it becomes available.

 

To enable the LMS failback feature: 

 

CLI: 

 

(c1-22) (AP system profile "default") #lms-preemption ? <cr>

(c1-22) (AP system profile "default") #lms-hold-down-period ? <lms-hold-down-period> Amount of time the primary LMS must be available before APs switch back after a failover Range: 1-3600. Default: 600.

 

WebUI:

 

rtaImage.png

 

 

 

Here's how the AP detects the primary LMS and fails back to it.

 

The AP periodically (every 60 seconds with random variation) sends a probe to the LMS. When it receives a response, it will set a MARK to current time. When the AP receives a subsequence response, the AP calculates the delta (current time minus MARK) and compares it with the configured holddown timer. If the delta is equal or greater than the holddown, the AP will bootstrap to the primary LMS. Otherwise, the AP will keep sending probes. If no response is received 10 seconds after the probe is sent, the AP will increment a counter (initially set to 0). If the counter reaches 3, the MARK is cleared. A new MARK is set when the AP receives a response.

 

 

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