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In order to manage IP multicasting, allow directed switching of multicast traffic, and dynamically configure switch ports so that IP multicast traffic is forwarded only to the appropriate ports, Cisco switches use:

Internet Group Management Protocol (IGMP) - A standard protocol designed to manage the multicast transmissions passed to routed ports by dynamically registering individual hosts in a multicast group. Hosts identify group memberships by sending IGMP messages to their local multicast routers. Under IGMP, routers listen to IGMP messages and periodically send out queries to discover which groups are active or inactive on a particular subnet. One of the problems with this protocol is if a VLAN on a switch is set to receive, all the workstations on that VLAN will get the multicast stream.

Cisco Group Management Protocol (CGMP) - A Cisco proprietary protocol designed to control the flow of multicast streams to individual VLAN port members while limiting the impact on the switch. CGMP requires IGMP to be running on the router.

IGMP

There are two versions of IGMP. Version 1 is defined in RFC 1112 and provides just two different types of IGMP messages:

Membership Reports - Hosts send out IGMP Membership Reports corresponding to a particular multicast group to indicate they are interested in joining that group.

Membership Queries - The router periodically sends out an IGMP Membership Query to verify that at least one host on the subnet is still interested in receiving traffic directed to that group. When there is no reply to three consecutive IGMP Membership Queries, the router will stop forwarding traffic directed toward that group. IGMP Version 2 is defined in RFC 2236.The primary difference is the inclusion of a Leave Group message, which allows hosts to take the initiative and actively communicate to the local multicast router that they no longer wish to be part of the multicast group. The router then sends out a group specific query and determines if there are any remaining hosts interested in receiving the traffic. If there are no replies, the router will time out the group and stop forwarding the traffic. This can greatly reduce the leave latency found with IGMP Version 1.

The default behavior for a Layer 2 switch would be to forward all multicast traffic to every port that belongs to the destination LAN on the switch. Basically, if one host on a VLAN wants to see the multicast, everybody on the VLAN gets it. Since the purpose of a switch is to limit traffic to just the ports that need to see it, this is not a desirable behavior. There are two methods to deal the problem - Cisco Group Management Protocol (CGMP) and IGMP Snooping.

CGMP

CGMP and IGMP software components run on both the Cisco routers and Cisco Catalyst switches. Together they allow these switches to leverage IGMP information on Cisco routers to make layer-2 (switching) forwarding decisions. With CGMP, IP Multicast traffic is delivered only to those Catalyst switch ports that are interested in the traffic; ports that have not explicitly requested the traffic will not receive it.

When the CGMP/IGMP-capable router receives an IGMP control packet, it processes it as it would any other IGMP request, and then creates a CGMP message, which it then forwards to the switch. These can either be “join” or “leave” messages, depending on what the host is asking for.

The switch receives the CGMP message and then modifies the port status in its CAM (Content Addressable Memory) table for that multicast group. All subsequent traffic directed to this multicast group will be forwarded to the port. The router port is also added to the entry for the multicast group.

It’s important to note that Multicast routers are required to monitor all multicast traffic for every group, since the IGMP control messages look just like regular multicast traffic. With CGMP, the switch only has to listen to CGMP “Join” and “Leave” messages from the router. The rest of the multicast traffic is forwarded using its CAM table as normal. The router carries the load.

Please note that if there is a spanning-tree topology change, the CGMP/IGMP-learned multicast groups on the VLAN are purged and the CGMP/IGMP-capable router must generate new multicast group information. If a CGMP/IGMP- learned port link is disabled, the corresponding port is removed from any multicast group.

CGMP/IGMP-capable routers send out periodic multicast group queries, so if a host wants to remain in a multicast group, it must respond to the query. If, after a number of queries, the router receives no reports from any host in a multicast group, the router sends a CGMP/IGMP command to the switch to remove the group from the forwarding tables. CGMP’s fast-leave-processing allows the switch to detect IGMP version-2 leave messages sent to the all- routers multicast address by hosts on any of the supervisor engine module ports.

Remember that CGMP must be configured on both the multicast routers and the layer-2 switches and that CGMP is Cisco proprietary.