Introduction

The CAPWAP standard for centralized WLAN management offers two different channels for transmissions:

The obligatory control channel transports administrative data between the managed AP and the WLC.
The optional data channel transmits the payload data from the various WLAN networks (SSID) between the managed AP and the WLC.

The decision whether to use of the optional data channel between the managed AP and the WLC depends on the route to be taken by the payload data:

If you deactivate the data channel, the AP 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 WLC and on the network as a whole, because the AP 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 AP additionally forwards the payload data to the central WLC. This approach has the following advantages:
- The APs can provide access to networks that are only available on the WLC, such as a central Internet access for a Public Spot.
- The WLANs provided by the APs (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 APs and in different IP networks can roam to other APs without interruption to their IP connections, because the connection is continually managed by the central WLC and not by the APs (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.

Figure 1. Overlay network across multiple IP networks

Using the data channel even allows you to span logical overlay networks across multiple WLCs.

Several WLCs within a single broadcast domain can support the same overlay network. Disable the WLC data channel between these WLCs (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 drop broadcast messages, you can activate the CAPWAP data channel for WLCs in separate networks.

The APs use virtual WLC interfaces (WLC tunnels) to manage each SSID's data channels between AP and WLC. Depending on the model, each WLC provides 16 to 32 WLC tunnels that you can use when configuring the logical WLANs.

Note: Virtual WLC interfaces are available for selection in all dialogs used to select logical interfaces (LAN or WLAN), such as in the port table of the LAN and VLAN settings or for the definition of IP networks.