Information About Multicast Optimization - Cisco Catalyst 3850 series Configuration Manual

Configuring Wireless Multicast
When the multicast mode is enabled and the device receives a multicast packet from the wired LAN, the
device encapsulates the packet using CAPWAP and forwards the packet to the CAPWAP multicast group
address. The device always uses the management VLAN for sending multicast packets. Access points in the
multicast group receive the packet and forward it to all the BSSIDs mapped to the VLAN on which clients
receive multicast traffic.
The device supports all the capabilities of v1 including Multicast Listener Discovery (MLD) v1 snooping but
the v2 and v3 capabilities are limited. This feature keeps track of and delivers IPv6 multicast flows to the
clients that request them. To support IPv6 multicast, global multicast mode should be enabled.
Internet Group Management Protocol (IGMP) snooping is introduced to better direct multicast packets. When
this feature is enabled, the device snooping gathers IGMP reports from the clients, processes them, creates
unique multicast group IDs (MGIDs) based on the Layer 3 multicast address and the VLAN number, and
sends the IGMP reports to the IGMP querier. The device then updates the access point MGID table on the
access point with the client MAC address. When the device 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 VLAN.
MGID is a 14-bit value filled in the 16-bit reserved field of wireless information in CAPWAP header. The
remaining 2 bits should be set to zero.
Related Topics
Configuring Wireless Multicast-MCMC Mode (CLI), on page 246
Configuring Wireless Multicast-MCUC Mode (CLI), on page 247

Information About Multicast Optimization

Multicast used to be based on the group of the multicast addresses and the VLAN as one entity, MGID. With
the VLAN group, duplicate packets might increase. Using the VLAN group feature, every client listens to
the multicast stream on a different VLAN. As a result, the device creates different MGIDs for each multicast
address and VLAN. Therefore, in a worst case situation, the upstream router sends one copy for each VLAN,
which results in as many copies as the number of VLANs in the group. Because the WLAN remains the same
for all clients, multiple copies of the multicast packet are sent over the wireless network. To suppress the
duplication of a multicast stream on the wireless medium between the device and the access points, the
multicast optimization feature can be used.
Multicast optimization enables you to create a multicast VLAN that can be used for multicast traffic. One of
the VLANs in the device can be configured as a multicast VLAN where multicast groups are registered. The
clients are allowed to listen to a multicast stream on the multicast VLAN. The MGID is generated using the
mulicast VLAN and multicast IP addresses. If multiple clients on different VLANs of the same WLAN are
listening to a single multicast IP address, a single MGID is generated. The device makes sure that all multicast
streams from the clients on this VLAN group always go out on the multicast VLAN to ensure that the upstream
router has one entry for all the VLANs of the VLAN group. Only one multicast stream hits the VLAN group
even if the clients are on different VLANs. Therefore, the multicast packets that are sent out over the network
is just one stream.
Related Topics
Configuring IP Multicast VLAN for WLAN (CLI), on page 254
OL-32598-01
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
Information About Multicast Optimization
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