Cisco Systems 3.2 manual Configuring Multicast Mode, Understanding Multicast Mode

Chapter 4 Configuring Controller Settings

Configuring Multicast Mode

Configuring Multicast Mode

If your network supports packet multicasting you can configure the multicast method that the controller uses. The controller performs multicasting in two modes:

•Unicast mode—In this mode the controller unicasts every multicast packet to every access point associated to the controller. This mode is inefficient but might be required on networks that do not support multicasting.

•Multicast mode—In this mode the controller sends multicast packets to an LWAPP multicast group. This method reduces overhead on the controller processor and shifts the work of packet replication to your network, which is much more efficient than the unicast method.

Understanding Multicast Mode

When you enable multicast mode, the controller does not become a member the multicast group. When the controller receives a multicast packet from the wired LAN, the controller encapsulates the packet using LWAPP and forwards the packet to the LWAPP multicast group address. The controller always uses the management interface for sending multicast packets. Access points in the multicast group receive the packet and forward it to all the BSSIDs mapped to the interface on which clients receive multicast traffic. From the access point perspective, the multicast appears to be a broadcast to all SSIDs.

When the source of the multicast is a wireless client, the multicast packet is unicast to the controller. In this case the controller makes two copies of the packet. One copy is the raw Ethernet packet that the controller sends out to the interface for the wireless LAN on which the client is associated, enabling the receivers on the wired LAN to receive the multicast traffic. The second copy of the packet is LWAPP-encapsulated and is sent to the multicast group. In this case the source of the multicast also receives the multicast packet, which helps the wireless client receive the multicast source.

Guidelines for Using Multicast Mode

Follow these guidelines when you enable multicast mode on your network:

•The Cisco Unified Wireless Network solution uses some IP address ranges for specific purposes, and you should keep these ranges in mind when configuring a multicast group:

–224.0.0.0 through 224.0.0.255—Reserved link local addresses

–224.0.1.0 through 238.255.255.255—Globally scoped addresses

–239.0.0.0 through 239.255.255.255—Limited scope addresses

•When you enable multicast mode on the controller you also must configure an LWAPP multicast group address on the controller. Access points subscribe to the LWAPP multicast group using IGMP.

•Cisco 1100, 1130, 1200, 1230, and 1240 access points use IGMP versions 1, 2, and 3. However, Cisco 1000 series access points use only IGMP v1 to join the multicast group.

•Multicast mode works only in Layer 3 LWAPP mode.

•Access points in monitor mode, sniffer mode, or rogue detector mode do not join the LWAPP multicast group address.

Cisco Wireless LAN Controller Configuration Guide

OL-8335-02 4-9