In lightweight mode, a Cisco access point can’t provide a working BSS for wireless users.
In order for a Cisco wireless access point in lightweight mode to become functional, it must join a Cisco Wireless LAN Controller to become active.
This dependency on a Cisco WLC is known as a split-MAC architecture: The access point handles most of the 802.11 processes and the Cisco WLC performs the management functions.
The lightweight access point joins the wireless LAN controller using a CAPWAP tunnel that extends through the network between the two devices. Control and data traffic are transmitted through this tunnel.
Multiple access points can join the same wireless LAN controller with it’s own CAPWAP tunnel. The wireless network can scale easily as long as the wireless LAN controller can support the number of lightweight access points.
Multiple different types of topologies can be constructed with lightweight access points.
If a wireless LAN controller is placed near to the core of the network, this is known as a centralised or unified wireless LAN topology.
A centralised or unified wireless LAN topology is ideal as it places users as close to the core or datacentre of the network, immediately in reach of frequently accessed resources.
Scalability can be an important factor with this design, especially in an enterprise network where there might dozens of lightweight access points.
Typically a Cisco wireless LAN controller can offer support for up to six thousand access points.
Where a CAPWAP tunnel is used with the lightweight access points, the VLANs offered to a wireless network only need to exist near the wireless LAN controller; and not the access points themselves.
Each lightweight access point will still contain it’s own management IP address that is obtained through communicating on the access port connected to the wireless access point.
A centralised architecture provides wireless mobility to users. As a user moves around different coverage areas of access points they may associate to a different access point. As the access point is joined to a single wireless LAN controller, they can easily have their connectivity maintained through the intelligence of the wireless LAN controller.
Two wireless users are treated different in a lightweight configuration in comparison to a autonomous configuration. Where a wireless to wireless communication may occur, the traffic will need to route back to the wireless LAN controller and back again to the other wireless user – even if they are connected to the same access point.
This type of communication is important why a wireless LAN controller should be located centrally; if it is not located centrally and connected to an access switch somewhere far away, the latency and bandwidth constrictions to get to that wireless LAN controller and back again for two different users may cause the connection experience for the users to suffer.
However there is a topology where it can be suited for a wireless LAN controller to be suited on the access layer.
Embedded Wireless Topology
Where a switch can support an embedded wireless LAN controller on its own platform, hosting an embedded wireless topology
An access point connects a CAPWAN tunnel to the same access switch it is located on; meaning the tunnel length is really short in comparison to a centralised point in the network architecture.
The embedded wireless topology can be a cost effective topology as there is no expense on a separate wireless LAN controller.
The embedded topology can solve issues at a branch site by providing a full functional wireless LAN controller built into a local switch at the site. They do not need to tunnel through to the centralised core wireless LAN controller located at the HQ for example.
The embedded topology also allows wireless to wireless communication to occur more effectively, due to there not being a path to the centralised core and back again.
Mobility Express Topology
The wireless LAN controller can be moved below the access layer switch and into an access point itself.
An access point acts as a wireless LAN controller for other lightweight access points, forming a CAPWAP tunnel between them.
This can be useful for multiple small sites for an example.