- The obligatory control channel transports administrative data between the managed access point and the WLAN controller.
- The optional data channel transmits the payload data from the various WLAN networks (SSID) between the managed access point and the WLAN controller.
The decision whether to use of the optional data channel between the managed access point and the WLAN controller depends on the route to be taken by the payload data:
- If you deactivate the data channel, the access point forwards the payload data directly to the LAN. In this case, you control the allocation of WLAN clients to specific LAN segments, for example by assigning VLAN IDs. The advantage of this application lies in the low load on the controller and on the network as a whole, because the access point transmits only the management data via the CAPWAP tunnel and it transmits the payload data over the shortest available route.
- If you activate the data channel, the access point additionally forwards the payload data
to the central WLAN controller. This approach has the following advantages:
- The access points can provide access to networks that are only available on the WLAN controller, such as a central Internet access for a Public Spot.
- The WLANs provided by the access points (SSIDs) can be separated from one another without the use of VLAN. Avoiding the use of VLAN reduces the effort required for the configuration of other network components such as switches, etc.
- WLAN clients associated with the access points and in different IP networks can roam to other access points without interruption to their IP connections, because the connection is continually managed by the central controller and not by the access points (layer-3 roaming).
The use of data channels forms additional logical networks on the basis of the existing physical infrastructure. These logical networks are known as overlay networks.
Overlay network across multiple IP networks
Using the data channel even allows you to span logical overlay networks across multiple WLAN controllers.
Several WLCs within a single broadcast domain can support the same overlay network. Disable the WLC data channel between these controllers (WEBconfig: LCOS Menu Tree > Setup > WLAN-Management > WLC-Cluster > WLC-Data-Tunnel-active). Otherwise the multiple reception of the broadcast messages would give rise to loops. Since routers discard broadcast messages, you can activate the CAPWAP data channel for controllers in separate networks.
The access points use virtual WLC interfaces (WLC tunnels) to manage each SSID's data channels between access point and WLAN controller. Depending on the model, each WLAN controller provides 16 to 32 WLC tunnels that you can use when configuring the logical WLANs.