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A broadcast address enables a device to send a datagram to all hosts on a subnetwork.
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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”
on page 41 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
3 on page 4 describes the function of each protocol that the router supports.
Table 3: Function of Multicast Protocols on a Router
FunctionProtocol
Discovers hosts that belong to multicast group.Internet Group Membership Protocol (IGMP)
Discovers other multicast routers to receive
multicast packets.
Protocol Independent Multicast Protocol (PIM)
Routes multicast datagrams within
autonomous systems.
Distance Vector Multicast Routing Protocol
(DVMRP)
Routes multicast datagrams between
autonomous systems.
BGP Multicasting Protocol
The router supports up to 16,384 multicast forwarding entries (multicast routes) at any
time.
Reverse-Path Forwarding
IP multicasting uses reverse path forwarding (RPF) to verify that a router receives a
multicast packet on the correct incoming interface. The RPF algorithm enables a router
to accept a multicast datagram only on the interface from which the router sends a
unicast datagram to the source of the multicast datagram.
When the router receives a multicast datagram from a source for a group, the router
verifies that the packet was received on the correct RPF interface. If the packet was not
Copyright © 2010, Juniper Networks, Inc.4
JunosE 11.2.x Multicast Routing Configuration Guide
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