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IPv4 Multicast Overview

IPv4 defines three types of addresses: unicast, broadcast, and multicast. Each type of address enables a device to send datagrams to selected recipients:

A unicast address enables a device to send a datagram to a single recipient.
A broadcast address enables a device to send a datagram to all hosts on a subnetwork.
A multicast address enables a device to send a datagram to a specified set of hosts, known as a multicast group, in different subnetworks.

Multicast IP packets contain a class D address in the Destination Address fields of their headers. A class D address is the IP address of a multicast group. See Configuring IGMP and JUNOSe IP, IPv6, and IGP Configuration Guide, for information about class D addresses.

IP multicast improves network efficiency by enabling a host to transmit a datagram to a targeted group of receivers. For example, for a host to send a large video clip to a group of selected recipients would be time-consuming to unicast the datagram to each recipient individually. If the host broadcasts the video clip throughout the network, network resources are not available for other tasks. The host uses only the resources it needs when multicasting the datagram.

Routers use multicast routing algorithms to determine the best route and transmit multicast datagrams throughout the network. E-series routers support a number of IP multicast protocols on virtual routers (VRs). Each VR handles the interoperability of IP multicast protocols automatically. To start multicast operation on a VR, you access the context for that VR and configure the desired protocols on the selected interfaces. Table 4 describes the function of each protocol that the router supports.

Table 4: Function of Multicast Protocols on a Router

Protocol	Function
Internet Group Membership Protocol (IGMP)	Discovers hosts that belong to multicast group.
Protocol Independent Multicast Protocol (PIM)	Discovers other multicast routers to receive multicast packets.
Distance Vector Multicast Routing Protocol (DVMRP)	Routes multicast datagrams within autonomous systems.
BGP Multicasting Protocol	Routes multicast datagrams between autonomous systems.

The router supports up to 16,384 multicast forwarding entries (multicast routes) at any time.

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