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How does scalability come into play with IP multicast?

Used for selectively transmitting information to multiple receivers, IP multicast is an efficient, bandwidth-conserving technology with inherent scalability and performance advantages. Multicast transmission eliminates the need to duplicate packets over the same link. IP multicast protocols have also been fine-tuned to create further networking efficiencies. The latest version of Internet Group Management Protocol, IGMPv3, supports source filtering, which allows end stations to receive data only from specified multicast sources, rather than from all sources. PIM-SM, a popular multicast routing protocol used in WANs, constructs a shared distribution tree, known as the RPT, to deliver multicast traffic to its destinations. To prevent unnecessary flooding, PIM-SM requires that routers explicitly join the RPT before they can receive multicast traffic for specific groups. PIM-SM also allows routers to switch to a more direct shortest path tree (SPT) if delivering traffic through the RPT introduces too much latency.

Multicast scalability refers to a device's ability to support an ever-increasing number of multicast sources and destinations, while processing a high rate of IGMP and PIM-SM control messages. Since the control plane and data plane compete for router resources, the number of multicast groups a router can support while still meeting forwarding performance expectations is an important scalability metric. Because a multicast-enabled router must be able to handle unicast and multicast traffic at the same time, mixed-class throughput is also a key measurement. RPT-to-SPT switchover delay, which measures how long it takes a router to construct and forward traffic through an SPT, is another important factor — especially when many sources send multicast traffic.