Using Multicast in Your Network
19-2 Configuring Multicast
2. Enabling the multicast protocol(s) on configured interfaces.
– For PIM, you must also configure a unicast routing protocol, such as OSPF.
– For both DVMRP and PIM-SM for IPv4 to operate, IGMP must be enabled.
Multicast Operation
Multicast allows a source to send a single copy of data using a single IP address from a well-
defined range for an entire group of recipients (a multicast group). A source sends data to a
multicast group by simply setting the destination IP address of the datagram to be the multicast
group address. Sources do not need to register in any way before they can begin sending data to a
group, and do not need to be members of the group themselves. Routers between the source and
recipients use the group address to route the data, forwarding duplicate data packets only when
the path to recipients diverges.
Hosts that wish to receive data from the multicast group join the group by sending a message to a
multicast router on a local interface, using a multicast group membership discovery protocol, such
as IGMP (IPv4). For more information, see “Internet Group Management Protocol (IGMP)” on
page 19-2.
Multicast routers communicate among themselves using a multicast routing protocol, such as
DVMRP or PIM-SM. These protocols calculate a multicast distribution tree of recipients to ensure
that:
• Multicast traffic reaches all recipients that have joined the multicast group
• Multicast traffic does not reach networks that do not have any such recipients (unless the
network is a transit network on the way to other recipients)
• The number of identical copies of the same data flowing over the same link is minimized.
For more information, see “Protocol Independent Multicast (PIM)” on page 19-11.
Internet Group Management Protocol (IGMP)
Overview
Group membership management is fundamental to the multicasting process. An arbitrary group
of receivers can express interest in receiving a particular multicast stream, regardless of the
physical or geographical boundaries of its members.
The purpose of IP multicast group management is to optimize a switched network’s performance
so multicast packets will only be forwarded to those ports containing multicast group hosts or
multicast switch devices instead of flooding to all ports in the subnet (VLAN).
IGMP uses three key components to control multicast membership:
• Source — A server that sends an IP multicast data stream with a particular multicast
destination IP and MAC address. A server may not have direct IGMP involvement, as it often
does not receive a multicast stream, but only sends a multicast stream.
• Querier — A device that periodically sends out queries in search of multicast hosts on a
directly connected network. If multiple queriers are present on the LAN, the querier with the
lowest IP address assumes the role.
• Host — A client end station that sends one of two IGMP messages to a querier:
– Join message — Indicates the host wants to receive transmissions associated to a
particular multicast group.
– Leave message — Indicates the host wants to stop receiving the multicast transmissions.
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