The SSID needs to be consistent for a wireless client to roam between LWAPs that are managed by the same WLC. However, if the LAPs are managed by different WLCs, then the Mobility group must be same on the WLCs. A Mobility Group is a group of Wireless LAN Controllers (WLCs) in a network with the same Mobility Group name. These WLCs can dynamically share context and state of client devices, WLC loading information, and can also forward data traffic among them, which enables inter-controller wireless LAN roaming and controller redundancy. Note that the WLCs may be in the same or different IP subnet or VLAN. WLCs use what is known as Ether-IPtunnel to transfer user traffic from one WLC to another.
Assuming that a User (or Client) originally joined the WLAN on WLC1, WLC1 will always refer to itself as the User's anchor point. Any controller that is serving the user from a different subnet is known as a foreign agent. As the client continues to roam, the anchor WLC will follow its movement by shifting the Ether-IP tunnel to connect with the User's foreign WLC.
In order for a wireless client to seamlessly roam between mobility group members (WLCs), WLAN's SSID and security configuration must be configured identically across all WLCs comprising the mobility group.
Advantages of Wireless LAN Controllers:
1. Centralized management of Access Points and the WLAN network.
2. Application of system-level mobility policies
3. Ease of relocating/replacing/installing a new AP in a wireless LAN network.
Disadvantage of Wireless LAN Controllers:
1. If a WLC (Wireless LAN Controller) fails, all the Aps connected to it will become in-operative. However, this may be overcome by providing fail-over WLC.