In typical deployment scenarios, each WLAN is mapped to a single dynamic interface per WLC, but consider a deployment scenario where there is a 4404-100 WLC that supports the maximum number of APs (100). Now consider a scenario where 25 users are associated to each AP. That would result in 2500 users who share a single VLAN. Some customer designs can require substantially smaller subnet sizes. One way to deal with this is to break up the WLAN into multiple segments. The AP grouping feature of the WLC allows a single WLAN to be supported across multiple dynamic interfaces (VLANs) on the controller. This is done when a group of APs is mapped to a specific dynamic interface. APs can be grouped logically by employee workgroup or physically by location.
AP Group VLANs are used in a setup where a Universal WLAN (service set identifier [SSID]) is required but clients need to be differentiated (placed on different interfaces configured on the WLC) by virtue of physical LAPs they associate with.
AP Group VLANs, also called Site-Specific VLANs, is a way to allow load balancing on a WLAN by creating groups of Cisco LAPs that override the interface normally provided by the WLAN. When a client joins a WLAN, the interface used is determined by the LAP it is associated with, and by looking up the AP Group VLAN and WLAN for that LAP.
The traditional method of assigning an interface to a device is based on the SSID or AAA policy override. In this case, if a client wants to broadcast information to another client on a WLAN, the broadcast is received by all the clients on that WLAN irrespective of whether it was intended for them or not.
The AP Group VLANs feature is an additional method used to limit the broadcast domains to a minimum. This is done by logically segmenting a WLAN into different broadcast domains. It limits the broadcast of a WLAN to a smaller group of LAPs. This helps to manage load balancing and bandwidth allocation more effectively. The AP Group VLANs feature creates a new table in the controller which lists the interfaces for every WLAN ID. Each entry in the table is indexed using a location name (which defines the group of LAPs).
Note: AP groups do not allow multicast roaming across group boundaries. AP groups allow APs on the same controller to map the same WLAN (SSID) to different VLANs. If a client roams between APs in different groups, the multicast session does not function properly because this is currently not supported. Currently, the WLC forwards multicast only for the VLAN configured on the WLAN and does not take into consideration VLANs configured in AP groups.
This list shows the maximum number of AP groups that you can configure on a WLC:
A maximum of 50 access point groups for the Cisco 2100 Series Controller and controller network modules
A maximum of 300 access point groups for the Cisco 4400 Series Controllers, Cisco WiSM, and Cisco 3750G Wireless LAN Controller Switch
A maximum of 500 access point groups for Cisco 5500 Series Controllers
Also refer to the link for more information-
http://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-vlan/71477-ap-group-vlans-wlc.html
