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Understanding Access Point Bias for Controllers


In wireless networks, distributed access points can have a bias for different controllers, which means they can have a preference for some controllers. Bias is used only when access points are distributed over the network, not when access points are directly plugged into a configured port on a controller. Bias does not effect the initial controller discovery process performed by the access point. Instead, for distributed access points only, bias is referenced by the controllers and the first controller an access point finds relays the bias settings to an access point.

This topic describes:

How Do I Determine the Bias Settings I should Use?

Refer to Table 1 to determine bias settings for access points.

Table 1: Configuring Bias on Controllers and Access Points


What You Should Do


Access point is directly wired and connected to a controller. Controller ports are configured as direct attach ports.

Bias settings are ignored in this case. You do not need to configure the access point bias. If you do configure it, that configuration is ignored.

The access point associates directly to the connected controller. An access point always attempts to boot on access point port 1 first, and if a controller is directly attached on access point port 1, the access point boots from there regardless of the bias settings.

There is one controller on the network.

Configure access points as usual. Bias will have no effect since there is only one controller.

Access points will associate to the controller, using one of these methods to discover the controller: static configuration, DHCP option 43, DNS, or subnet directed broadcast.

There are three controllers on the network, set up as shown in Figure 1.

In this situation, bias will make a difference. The access point has a high bias for WLC1 and WLC3, and a low bias for WLC2.

Since the controllers are in another subnet, the access points locate a controller and receive the bias settings and the capacity of each controller.

Access points are then directed to associate to a high bias controller (WLC1 or WLC3 in this case) if one is available .

When the bias for two controllers is the same, as it is in this illustration, the controller with the greatest capacity to add more active access points is selected.

Example of a Layer 3 Network With Multiple Controllers

This Layer 3 network would make use of bias settings since there are several controllers that access points can associate to.

Figure 1: Controllers With High Bias and Low Bias for an Access Point
Controllers With High Bias and
Low Bias for an Access Point

A Third Option for Access Point Bias: Sticky

Access points actually have three bias options for controllers, high, low, and sticky. The third option, sticky, means that an access point stays on whatever controller it is associated to, regardless of whether a high bias controller is available or not. Even if a sticky access point is using a low bias controller when a high bias controller becomes available, the sticky access point stays with the low bias controller. The sticky option is useful because it avoids unscheduled access point resets that would normally occur as the access point transitions from one controller to another.

An access point with a sticky bias stays on the controller until an administrator intervenes by resetting the access point to have it discover and connect to a high bias controller.

How Do I Set the Controller Bias for an Access Point?

When you add an access point, you must first select a controller—this controller is the one you are setting the bias for. If you want the access point to be able to use different controllers, you must set the bias for each controller. You would add the access point to each controller following the directions Adding and Managing an Individual Access Point.

How Can I Determine the Bias of an Access Point by Looking at a Controller?

If an access point is associated with a controller, it appears on the list you see when you click Wireless Network > View Inventory (under Device Management) in Network Director. If the access point is using that controller, the Connection field on the controller will indicate Up. If the controller’s Connection field indicates Up/Redundant, it means that cluster/network resiliency is enabled. In that case, each access point has a primary link to one controller and a backup/redundant link to another controller. The redundant controller connection is also listed as Up/Redundant.

What About Controller Clusters?

Clusters are a subset of a mobility domain. Clustering creates a logical group of controllers (and their associated access points) that share network and user information for failover support. Every access point in a cluster has a primary access point manager—this can be any controller in the cluster. For more information, see Creating a Mobility Domain for Wireless LAN Controllers.

The concept of bias is superceded in cluster configuration scenarios by automatic redundancy and/or optional access point affinity configuration. Access point affinity is configured per controller in a cluster, so bias has no bearing or effect.