N+1 High Availability (HA)

Since release 7.4 for Wireless LAN Controllers (WLC) more and more customers are using this solution to provide wireless redundancy. The HA-SKU secondary WLC within the Cisco Unified Wireless Network (CUWN) framework allows a single WLC to be used as a backup WLC for N primary controllers. The advantages of this would be cost (HA-SKU controller with no need for additional licences) and the secondary controller could be geographically separate from any of the other primary controllers.

N+1 HA

These WLCs are independent of each other and do not share configuration or IP addresses on any of their interfaces. Each WLC needs to be managed separately by Cisco Prime, can run a different hardware and a different software version (it would however make sense to have all WLC’s on the same software version) and can be deployed in different data centres across the WAN link.

When an AP fails over to a WLC running a version other than that on the primary, the corresponding image is downloaded to the AP. This adds to the failover time. Again, it is recommended to have your WLC’s on the same software version.

When a primary WLC resumes operation, the APs fall back from the backup WLC to the primary WLC automatically if the AP fallback option is enabled. AP’s with high priority on the primary controller always connect first to the backup controller, even if they have to push out low priority APs.

On the HA-SKU secondary controller the 90-day timer will start when the AP’s join the controller and the user will see a warning message after 90 days. In other words, an HA-SKU controller can be used as a secondary controller for 90 days without a warning message. Starting release 7.6, if all the access points fall back to the primary controller within or after the 90 days period, the timer will be reset and warning messages will stop.

The HA-SKU provides the capability of the maximum number of APs supported on that hardware. For instance, a 5508 HA- SKU controller provides support for 500 APs.

Configuration

From the primary controller, navigate to Access Points > Global Configuration, then configure the backup controller on the primary to point to the secondary controller.

On the secondary controller, navigate to Controller > Redundancy > Global Configuration, then configure the secondary controller to convert it to an HA-SKU secondary controller. Ensure Redundant Unit are changed to Secondary and AP SSO is Disabled.

On all WLC’s under Wireless > All APs > High Availability your HA-SKU can be configured as secondary or tertiary as needed.

Failover Process

In the N+1 HA redundancy model, one WLC serves as the backup controller for N primary controllers. When any of the primary WLCs fail, the APs connected to that controller fall back to the backup controller. The AP has to restart its CAPWAP state machine and go through a complete discovery phase before it joins the backup controller. The available AP count on the backup controller is reduced by the number of APs that fall back from the primary WLC to the backup WLC.

For example, when the primary controller supporting 90 APs fails, these APs fall back to the backup controller that has a maximum AP support of 500. The backup WLC is left with an available AP count of 500 – 90 = 410 APs.

Nice and easy!

Reference:

http://www.cisco.com/c/en/us/td/docs/wireless/technology/hi_avail/N1_High_Availability_Deployment_Guide/N1_HA_Overview.html

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N+1 High Availability (HA)

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