twhittle1 networking blog: Cisco Wireless N+1 HA

Here is a little scatter shot information about Cisco WLCs in the N+1 HA model:

When using N+1 HA make sure the controllers are running V7.4 or later. Pre 7.4 the N+1 HA model requires a permanent AP count license on the backup controller. With Release 7.4 and later, an HA-SKU secondary controller can be used as the backup controller for multiple primary controllers. The overall goal for the addition of N+1 HA with HA-SKU is to reduce the total cost of ownership (TCO) for geographically separate HA deployments across the WAN link.

Keep in mind that Access Point Stateful Switch Over (AP SSO) functionality is not supported for N+1 HA. The AP Control and Provisioning of Wireless Access Points (CAPWAP) state machine is restarted when the primary controller fails. With Release 7.4, the backup controller for N+1 HA can be an HA-SKU secondary controller. AIR-CT5760-HA-K9 is the HA SKU for the new Cisco 5760 WLC, for example.

an HA WLC, with a different user count to the main controllers, can be used as the HA controller. For example 2 x WISM2 each with 750 AP licenses can be backed up by a WLC5670 which could only support 1000 APs.

The new 5760 WLC does support N+1 High Availability. If interworking with AireOS controllers in the same Mobility group, you will need either a WiSM2 or a CT5508 running 7.3.112.0 release and also in "Hierarchal Mobility Mode" to form a Mobility group with IOS-XE based Controllers [3.2.0SE release]. Be aware that in the current release, when AP's fail between dissimilar WLC Operating Systems (AireOS and IOS-XE) the AP's do download a new CAPWAP image and reboot; so this failover is not "sub 45 seconds" but rather 2~3 minutes depending on download speeds and reboot times of Access Points.

References:

Cisco Catalyst 3850 Series Switches – Q&A

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps12686/qa_c67-722110.html

Cisco Catalyst 3850 Switch Deployment Guide

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps12686/deployment_guide_c07-727067.html

Cisco Unified Access Technology Overview: Converged Access

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps12686/white_paper_c11-726107.html

Converged Access Mode for the Cisco 5760 WLC and the Catalyst 3850 Switch

http://www.cisco.com/en/US/prod/collateral/wireless/ps6302/ps8322/ps12598/qa_c67-726507.html

Cisco Wireless Software Compatibility Matrix - (Converged Access WLC Compatibility Matrix – Table 6)

http://www.cisco.com/en/US/docs/wireless/controller/5500/tech_notes/Wireless_Software_Compatibility_Matrix.html#wp92761

Release Notes for Cisco WLC's and Lightweight Access Points