A Mobility Controller resides on the switch. It is both, control path and data path entity and is responsible for:
- Handling the mobility events on the switch
- Configuring the datapath elements on the switch for mobility, and
- Communicating with the mobility controller
As MA, the device performs the datapath functions by terminating the CAPWAP tunnels that encapsulate 802.11 traffic sourced by wireless stations.
This allows the device to apply features to wired and wireless traffic in a uniform fashion. As far as device is concerned, 802.11 is just another access medium.
The MA performs the following functions:
- Support the mobility protocol – The MA is responsible for responding in a timely manner, ensuring the device is capable of achieving its roaming budget.
- Point of presence – If the wireless subnets are not available at the MC, the MA assumes the point of presence if the wireless client VLAN is not available at the new point of attachment and tunnel the client traffic accordingly.
- ARP Server – When the network is configured in a layer 2 mode, the MA is responsible for advertising reachability for the stations connected to it. If tunneling is employed, the ARP request is transmitted on behalf of the station through the tunnel, which the point of presence (anchor switch) would bridge onto its uplink interface.
- Proxy IGMP – The MA on the device is responsible for subscribing to multicast groups on behalf of a station after a roaming event has occurred. This information is passed as part of the context to the new device. This ensures the multicast flows follow the user as it roams.
- Routing – When the device is connected to a layer 3 access network, the MA is responsible for injecting routes for the stations that are associated with it for which tunneling is not provided.
- 802.1X Authenticator – The authenticator function is included in the MA, and handles both wired and wireless stations.
- Secure PMK Sharing – When a station successfully authenticates to the network, the MA forwards the PMK to the MC. The MC is responsible for flooding the PMK to all the MAs under its sub-domain and to the peer MCs in the mobility group.
The MA also performs the following datapath functions:
- Mobility tunnel – If tunneling is used, the MA encapsulates and decapsulates packets from the mobility tunnel to the MC, and to other MA in the peer group, if the access switches are serving as points of presence. The MA supports the tunneling of client data traffic between the point of attachment and the point of attachment. The packet format used for other switches is CAPWAP with an 802.3 payload. The MA also supports reassembly and fragmentation for mobility tunnels.
Encryption – The mobility control traffic between the mobility nodes is DTLS encrypted. The MA also encrypts the CAPWAP control and data (optional) at the point of attachment.
CAPWAP – The device supports the CAPWAP control and data planes. The device forwarding logic is responsible for terminating the CAPWAP tunnels with 802.11 as well as 802.3 payloads. Since support for large frames (greater than 1500bytes) is not universally available, the device supports CAPWAP fragmentation and reassembly.
Mobility tunnel path via an L3 interface on the 4500 or the L3 interface on the uplink port is not supported. It is not possible to have an L3 wireless management interface. Even if the tunnel comes up, packet forwarding is not possible as it is not supported. 4510 drops DHCP packets from wireless clients if SSID is anchored to a different Cisco WLC.