When an AP is fully joined to a controller, the AP learns of all the controllers configured in that mobility group. Should the controllers that an AP is currently registered with go down, the AP will send discoveries to any and all controllers in the mobility group. Assuming one of the controller has the capacity to accept the AP, the AP should join the least loaded controller it can find. If many controllers in the mobility group, it can be difficult to determine what controller the APs will join should their current controller fail.

If you want to have more control over how the APs move between controllers on your network, you can configure the APs with Primary, Secondary & Tertiary controller names. With the controller name configured on APs, the APs always try to register the primary controller first. Should the primary controller go down, the AP tries to register with the secondary controller. If the AP is not able to join any of the configured controllers, it try to join any controller with Master Controller setting configured, or if no Master Controller, then the least loaded controller in the Mobility Group.

AP Failover priority can be used to determine who will register for a controller if there is  a contention. You can configure your wireless network so that the backup controller recognize a join request from a higher priority AP and if necessary disassociates a lower priority AP as  a means to provide an available port for higher fail over priority AP.

Below show the Primary, Secondary, Tertiary controller settings for a AP.(In High Availabilty tab of AP configuration)

You can configure the same via WLC CLI using following 3 commands.

config ap primary-base <controller_name> <Cisco_AP_name> [controller_IP_Addres]
config ap secondary-base <controller_name> <Cisco_AP_name> [controller_IP_Addre]
config ap tertiary-base <controller_name> <Cisco_AP_name> [controller_IP_Addres]

Here is an example of where I have configured primary,secondary, tertiary controller information for an AP named “1252-c”

(4402-a) >config ap primary-base 4402-a 1252-c 10.10.20.100
(4402-a) >config ap secondary-base 4402-b 1252-c 10.10.10.10
(4402-a) >config ap tertiary-base 4402-c 1252-c 10.10.10.20

Should you want to set a global primary backup and or secondary backup controller for all the APs joined to a particular controller. You can configure this through “Wireless -> All AP -> Global Configuration” section as shown in the below.You enable/disable Fast Heartbeat for local/H-REAP from the same page.

You can use CLI to configure this as well.

config advanced backup-controller primary <backup_controller_name> <IP_Address>
config advanced backup-controller secondary<backup_controller_name> <IP_Addres>
!
config advanced timers ap-fast-heartbeat {local|hreap|all} {ebable|disable} <interval 1-10>
config advanced timers ap-heartbeat-timeout <interval 1-30> (at least 3 times heartbeat timer)
config advanced timers ap-primary-discovery-timeout <interval 30-3600> (default it 120s)
config advanced timers ap-discovery-timeout <interval 1-10> (default is 10s)
config advanced timers auth-timeout <interval 10-600> (default is 10s)

Here is the same configuration shown in the above GUI screen capture, if you want to do it via CLI.

(4402-a) >config advanced backup-controller primary 4402-d 10.10.200.10
(4402-a) >config advanced backup-controller secondary 4402-c 10.10.10.20

Following CLI commands can be used to verify those settings.

(4402-a) >show advanced timers 
Authentication Response Timeout (seconds)........ 10
Rogue Entry Timeout (seconds).................... 1200
AP Heart Beat Timeout (seconds).................. 30
AP Discovery Timeout (seconds)................... 10
AP Local mode Fast Heartbeat (seconds)........... disable
AP Hreap mode Fast Heartbeat (seconds)........... disable
AP Primary Discovery Timeout (seconds)........... 120
AP Primed Discovery Timeout (seconds)............ 0

(4402-a) >show advanced backup-controller 
AP primary Backup Controller .................... 4402-d 10.10.200.10
AP secondary Backup Controller .................. 4402-c 10.10.10.20

(4402-a) >show ap config general 3502-d
Cisco AP Identifier.............................. 0
Cisco AP Name.................................... 3502-d
Country code..................................... Multiple Countries:AU,LK,NZ
Regulatory Domain allowed by Country............. 802.11bg:-AE    802.11a:-EN
AP Country code.................................. AU  - Australia
AP Regulatory Domain............................. 802.11bg:-A    802.11a:-N 
Switch Port Number .............................. 1
MAC Address...................................... 44:d3:ca:af:43:43
IP Address Configuration......................... Static IP assigned
IP Address....................................... 10.10.20.4
IP NetMask....................................... 255.255.255.0
Gateway IP Addr.................................. 10.10.20.1
Domain........................................... 
Name Server...................................... 
NAT External IP Address.......................... None
CAPWAP Path MTU.................................. 1485
Telnet State..................................... Disabled
Ssh State........................................ Disabled
Cisco AP Location................................ 3750-A Port4
Cisco AP Group Name.............................. default-group
Primary Cisco Switch Name........................ 4402-a
Primary Cisco Switch IP Address.................. 10.10.20.100
Secondary Cisco Switch Name...................... 4402-b
Secondary Cisco Switch IP Address................ 10.10.10.10
Tertiary Cisco Switch Name....................... 4402-c
Tertiary Cisco Switch IP Address................. 10.10.10.20
Administrative State ............................ ADMIN_ENABLED
Operation State ................................. REGISTERED
Mirroring Mode .................................. Disabled
AP Mode ......................................... Local
Public Safety ................................... Disabled 
AP SubMode ...................................... Not Configured
Remote AP Debug ................................. Disabled
Logging trap severity level ..................... informational
Logging syslog facility ......................... kern
S/W  Version .................................... 7.0.116.0
Boot  Version ................................... 12.4.2.4
Mini IOS Version ................................ 7.0.112.74
Stats Reporting Period .......................... 180
LED State........................................ Enabled
PoE Pre-Standard Switch.......................... Disabled
PoE Power Injector MAC Addr...................... Disabled
Power Type/Mode.................................. Power injector / Normal mode
Number Of Slots.................................. 2 
AP Model......................................... AIR-CAP3502I-N-K9   
AP Image......................................... C3500-K9W8-M
IOS Version...................................... 12.4(23c)JA2
Reset Button..................................... Enabled
AP Serial Number................................. FGL1533S1U8
AP Certificate Type.............................. Manufacture Installed
AP User Mode..................................... AUTOMATIC
AP User Name..................................... Not Configured
AP Dot1x User Mode............................... Not Configured
AP Dot1x User Name............................... Not Configured
Cisco AP system logging host..................... 255.255.255.255
AP Up Time....................................... 0 days, 15 h 37 m 09 s
AP LWAPP Up Time................................. 0 days, 14 h 19 m 15 s
Join Date and Time............................... Sun Apr  7 08:02:40 2013
Join Taken Time.................................. 0 days, 00 h 01 m 16 s
Ethernet Port Duplex............................. Auto
Ethernet Port Speed.............................. Auto
AP Link Latency.................................. Disabled
Rogue Detection.................................. Enabled
AP TCP MSS Adjust................................ Enabled
AP TCP MSS Size.................................. 1363

When using both the local (primary, secondary, tertiary) and global backup configurations, the locally configured settings take precedence in the event of a controller failure. If an AP is not able to join any of the locally configured controllers, it then tries to join the global backup controllers.

When an AP moves off the primary controller, it joins another controller and stays registered to that controller until the primary controller comes back online. The AP continuous to send primary discovery request every 30s to the configured primary controller. AP primary discovery timeout value is set to 120s by default (it can be value from 30s to 3600s). As soon as the primary controller responds, the AP tries to re-join it.

There may be a situation where APs do not move back to the configured primary controller when that controller is back on the network. When this happens, ensure that AP fallback is enabled under “Controller -> General” section.

If that setting is disabled, the AP remain on the backup controller until you manually reboot them. AP fallback is enabled by default.

To configure AP failover priority, you have to enable this feature globally (“Wireless -> All AP -> Global Configuration”) & then individual APs with a suitable priority level. By default all APs are set to priority level 1 ( Low). Other values are (2- Medium, 3- High, 4-Critical)

Here is the individual AP fail-over priority settings you can choose.

By using CLI you can configure AP fail-over priority as below. To enable this globally you can use “config network ap-priority {enable|disable}” command. Then specify the priority of an AP by entering this CLI command.
config ap priority {1|2|3|4} <Cisco_AP_name>

Here is an example of this CLI usage.

(4402-a) >config network ap-priority enable
(4402-a) >config ap priority 3 3502-d

You can verify this by “show network summary” & “show ap summary” CLI output as shown below.

(4402-a) >show ap summary 
Number of APs.................................... 2
Global AP User Name.............................. Not Configured
Global AP Dot1x User Name........................ Not Configured
AP Name             Slots  AP Model              Ethernet MAC       Location          Port  Country  Priority
------------------  -----  --------------------  -----------------  ----------------  ----  -------  ------
3502-d               2     AIR-CAP3502I-N-K9     44:d3:ca:af:43:43      3750-A Port4  1        AU       3
1252-c               2     AIR-LAP1252AG-N-K9    c8:4c:75:2c:95:c0      3750-a-PORT3  1        NZ       1

(4402-a) >show network summary 
RF-Network Name............................. mrn-rfg
Web Mode.................................... Disable
Secure Web Mode............................. Enable
Secure Web Mode Cipher-Option High.......... Disable
Secure Web Mode Cipher-Option SSLv2......... Enable
Secure Shell (ssh).......................... Enable
Telnet...................................... Disable
Ethernet Multicast Forwarding............... Enable
Ethernet Broadcast Forwarding............... Disable
AP Multicast/Broadcast Mode................. Multicast   Address : 239.239.239.1
IGMP snooping............................... Enabled
IGMP timeout................................ 60 seconds
IGMP Query Interval......................... 20 seconds
User Idle Timeout........................... 300 seconds
ARP Idle Timeout............................ 300 seconds
Cisco AP Default Master..................... Disable
AP Join Priority............................ Enabled
Mgmt Via Wireless Interface................. Disable
Mgmt Via Dynamic Interface.................. Disable
Bridge MAC filter Config.................... Enable
Bridge Security Mode........................ EAP
Mesh Full Sector DFS........................ Enable
Apple Talk ................................. Disable
AP Fallback ................................ Enable
Web Auth Redirect Ports .................... 80
Web Auth Proxy Redirect  ................... Disable
Fast SSID Change ........................... Disabled
802.3 Bridging ............................. Disable
IP/MAC Addr Binding Check .................. Enable

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