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Software Configuration Guide—Release 12.2(25)SG
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Chapter24 Understanding and Configuring IP Multicast
Overview of IP Multicast
At the other end of the IP communication spectrum is an IP broadcast, where a source host sends packets
to all hosts on a network segment. The destination address of an IP broadca st packet has the host portion
of the destination IP address set to all ones and the network portion set to the address of the subnet. IP
hosts, including routers, understand that packets, which contain an IP broadcast address as the
destination address, are addressed to all IP hosts on the subnet. Unless specifically configured otherwise,
routers do not forward IP broadcast packets, so IP broadcast communication is normally limited to a
local subnet.
IP multicasting falls between IP unicast and IP broadcast communication. IP multicast communication
enables a host to send IP packets to a group of hosts anywhere within the IP network. To send
information to a specific group, IP multicast communication uses a special form of IP destination
address called an IP multicast group address. The IP multicast group address is specified in the IP
destination address field of the packet.
To multicast IP information, Layer 3 switches and routers must forward an incoming IP packet to all
output interfaces that lead to members of the IP multicast group. In the multicasting process on the
Catalyst 4500 series switch, a packet is replicated in the Integrated Switchin g Engine, forwarded to the
appropriate output interfaces, and sent to each member of the multicast group.
It is not uncommon for people to think of IP multicasting and video conferencing as almost the same
thing. Although the first application in a network to use IP multicast is often video conferencing, video
is only one of many IP multicast applications that can add value to a company’s business model. Other
IP multicast applications that have potential for improving productivity include multimedia
conferencing, data replication, real-time data multicasts, and simulation applications.
This section contains the following subsections:
•IP Multicast Protocols, page 24-2
•IP Multicast on the Catalyst 4500 Series Switch, page 24-4
•Unsupported Features, page 24-12
IP Multicast Protocols
The Catalyst4500 series switch primarily uses these protocols to implement IP multicast routing:
•Internet Group Management Protocol (IGMP)
•Protocol Independent Multicast (PIM)
•IGMP snooping and Cisco Group Management Protocol
Figure 24-1 shows where these protocols operate within the IP multicast environment.