Due to the growing requirement for networks to support multimedia applications that must transmit the same video and voice information to many people, the need for multicast traffic has arisen. Multicast traffic provides one-to-many delivery, unlike one-to-one (unicast) and one-to-all (broadcast) traffic types. Multicast traffic control allows you to control where a multicast transmission is flooded. Ideally you want this transmission flooded only to receivers that have subscribed to the multicast.
To enable multicast on your network, you must support multicast at both the Layer 2 and Layer 3 level. From a Layer 3 perspective, a multicast network determines which router interfaces (segments) a multicast should be forwarded to. Multicast routing topologies are used to build the Layer 3 paths over which multicast traffic flows. PIM is an example of a common multicast routing protocol used today. It operates in three modes:
PIM dense mode uses a flood and prune mechanism to build SPTs between a source and receivers. PIM sparse mode uses a combination of shared trees and SPTs to eventually build an SPT between a source and receivers, without requiring the flooding mechanism used by PIM dense mode operation. A key component of PIM sparse mode is the RP, which is where all shared trees are rooted. PIM sparse-dense mode allows PIM routers to use PIM sparse mode when an RP is known for a group or PIM dense mode when no RP is known for a group. PIM sparse-dense mode enables RP discovery protocols, such as Auto-RP, by using PIM dense mode for Auto-RP messages, ensuring all multicast routers receive RP information.
Once a multicast leaves routers to transit a LAN segment, the multicast router has no control over the multicast. Instead of flooding the multicast out all ports on a switch, you can limit the multicast transmission to only hosts that are receivers of a particular group. The following dynamic mechanisms exist for controlling multicast traffic on the LAN:
IGMP snooping works by having the switch inspect IGMP traffic sent between a multicast router and receiving hosts. IGMP is the protocol used to maintain multicast group membership. By understanding which hosts belong to a particular group, the switch can flood the multicast group traffic only to those hosts.
CGMP (proprietary) works by having a multicast router relay IGMP messages to a LAN switch in a CGMP message. The multicast router provides notification of router and host joins and leaves.