Chapter 9
Multicast
Refer to the following sections to configure and use these features:
■ 9-1: Multicast Addressing: Describes multicast flows and multicast addressing in
relation to networking devices
■ 9-2: IGMP Snooping: Explains how to configure a switch to constrain multicast
traffic by listening to Internet Group Management Protocol (IGMP) messages
9-1: Multicast Addressing
Multicasts are directed flows in the network. Application servers send packets and mes-
sages to addresses that must be forwarded by network devices and then picked up by the
appropriate end devices. The network devices choose which links to forward based on
these addresses. The devices track flows from the servers. The list that follows summa-
rizes the characteristics of multicast:
■ IP multicast flows can be designated by these notations:
■ (S,G): A unique shortest path tree structure between the source and the multi-
cast destinations, pronounced “S comma G.” S is an IP unicast source address,
and G is the IP multicast destination address or group.
■ (*,G): A common shared tree structure, where a multicast rendezvous point (RP)
accepts multicast traffic from the source and then forwards it on to the destina-
tions, pronounced “Star comma G.” The star or asterisk (*) represents the RP
because it is a wildcard source that accepts input from any real multicast source.
The G represents the IP multicast destination address or group.
■ IP multicast or Class D addresses begin with 1110 in the most significant address
bits—Addresses within the range 224.0.0.0 to 239.255.255.255.
■ Hosts anywhere in the network can register to join a multicast group defined by a
specific multicast IP address. Registration is handled through the IGMP.
■ IP multicast addresses 224.0.0.1 (all hosts on a subnet) and 224.0.0.2 (all routers on a
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