2.37.1.14 Multicast-Networks

This table contains the settings for the transmission of CAPWAP multicast packets over the bridge interfaces.

When a WLC receives a broadcast or multicast packet from a network belonging to a certain SSID, then it has to forward this packet to all APs that work with that SSID. The WLC has two ways to reach all of these APs:

The WLC copies the packet and sends it as a unicast to the relevant APs. The replication of packets increases the CPU load on the controller and the necessary bandwidths, which negatively impacts performance especially of WAN connections.
The WLC sends the packet as a multicast. In this case, a single packet only has to be transmitted. However, multicast packets sent from a controller only reach those APs in its own broadcast domain. APs at the other end of a routed WAN link cannot receive multicast packets from the controller.
Note: The forwarding of multicast packets depends on the devices operated on the WAN route.

The WLC regularly sends keep-alive multicast packets to the multicast group. If an AP responds to these packets, the controller is able to reach this AP with multicast packets. For all other APs, the controller copies the multicast packets it receives and sends them as a unicast to the appropriate APs.

If the transmission of CAPWAP multicast packets has been activated and a valid multicast IP address with port has been defined for the bridge interface, the device forwards the incoming broadcast and multicast packets as a multicast to this address.

To ensure that the information about associated WLAN clients and their multicast group memberships is kept up to date even when they switch between APs, devices operating multicast simultaneously activate IGMP snooping for continuous updates to the information on multicast structure.

In applications featuring multiple WLCs, multicast packets can lead to loops. In order to avoid loops due to multicasts when using the bridge, the WLC applies the following measures:

The WLC ignores CAPWAP multicast packets. When working with a WLC data tunnel, the controller sends these packets as unicast.
The WLC does not forward packets that carry a CAPWAP multicast address as the recipient.
The WLC automatically enables IGMP snooping on all managed APs if CAPWAP works with multicast.