For example, suppose that there are two controllers with management interface IP addresses 10.126.126.2 and 10.127.127.2.
The type is f1(hex). The length is 2 * 4 = 8 = 08 (hex). The IP addresses translate to 0a7e7e02 and 0a7f7f02. Assembling
the string then yields f1080a7e7e020a7f7f02.
The resulting Cisco IOS command added to the DHCP scope is listed below:
option 43 hex f1080a7e7e020a7f7f02
Backup Controllers
A single controller at a centralized location can act as a backup for mesh access points when they lose
connectivity with the primary controller in the local region. Centralized and regional controllers need not be
in the same mobility group. Using the controller GUI or CLI, you can specify the IP addresses of the backup
controllers, which allows the mesh access points to fail over to controllers outside of the mobility group.
You can also configure primary and secondary backup controllers (which are used if primary, secondary, or
tertiary controllers are not specified or are not responsive) for all access points connected to the controller as
well as various timers, including the heartbeat timer and discovery request timers.
The fast heartbeat timer is not supported on access points in bridge mode. The fast heartbeat timer is
configured only on access points in local and FlexConnect modes.
Note
The mesh access point maintains a list of backup controllers and periodically sends primary discovery requests
to each entry on the list. When the mesh access point receives a new discovery response from a controller,
the backup controller list is updated. Any controller that fails to respond to two consecutive primary discovery
requests is removed from the list. If the mesh access point’s local controller fails, it chooses an available
controller from the backup controller list in this order: primary, secondary, tertiary, primary backup, and
secondary backup. The mesh access point waits for a discovery response from the first available controller in
the backup list and joins the controller if it receives a response within the time configured for the primary
discovery request timer. If the time limit is reached, the mesh access point assumes that the controller cannot
be joined and waits for a discovery response from the next available controller in the list.
When a mesh access point’s primary controller comes back online, the mesh access point disassociates
from the backup controller and reconnects to its primary controller. The mesh access point falls back to
its primary controller and not to any secondary controller for which it is configured. For example, if a
mesh access point is configured with primary, secondary, and tertiary controllers, it fails over to the tertiary
controller when the primary and secondary controllers become unresponsive and waits for the primary
controller to come back online so that it can fall back to the primary controller. The mesh access point
does not fall back from the tertiary controller to the secondary controller if the secondary controller comes
back online; it stays connected to the tertiary controller until the primary controller comes back up.
Note
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Connecting the Cisco 1500 Series Mesh Access Points to the Network
Backup Controllers
