Specifications

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Cisco Wireless LAN Controller Configuration Guide
OL-21524-02
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 a CAPWAP 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.
You can enable multicast mode using the controller GUI or CLI.
Understanding Multicast Mode
When you enable multicast mode and the controller receives a multicast packet from the wired LAN, the
controller encapsulates the packet using CAPWAP and forwards the packet to the CAPWAP 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.
In controller software release 4.2 or later releases, Internet Group Management Protocol (IGMP)
snooping is introduced to better direct multicast packets. When this feature is enabled, the controller
gathers IGMP reports from the clients, processes them, creates unique multicast group IDs (MGIDs)
from the IGMP reports after selecting the Layer 3 multicast address and the VLAN number, and sends
the IGMP reports to the infrastructure switch. The controller sends these reports with the source address
as the interface address on which it received the reports from the clients. The controller then updates the
access point MGID table on the access point with the client MAC address. When the controller receives
multicast traffic for a particular multicast group, it forwards it to all the access points, but only those
access points that have active clients listening or subscribed to that multicast group send multicast traffic
on that particular WLAN. IP packets are forwarded with an MGID that is unique for an ingress VLAN
and the destination multicast group. Layer 2 multicast packets are forwarded with an MGID that is
unique for the ingress interface.
When IGMP snooping is disabled, the following is true:
• The controller always uses Layer 2 MGID when it sends multicast data to the access point. Every
interface created is assigned one Layer 2 MGID. For example, the management interface has an
MGID of 0, and the first dynamic interface created is assigned an MGID of 8, which increments as
each dynamic interface is created.
• The IGMP packets from clients are forwarded to the router. As a result, the router IGMP table is
updated with the IP address of the clients as the last reporter.
When IGMP snooping is enabled, the following is true:
• The controller always uses Layer 3 MGID for all Layer 3 multicast traffic sent to the access point.
For all Layer 2 multicast traffic, it continues to use Layer 2 MGID.
• IGMP report packets from wireless clients are consumed or absorbed by the controller, which
generates a query for the clients. After the router sends the IGMP query, the controller sends the
IGMP reports with its interface IP address as the listener IP address for the multicast group. As a
result, the router IGMP table is updated with the controller IP address as the multicast listener.