IPv4 multicast address conventions are described in the following sections. IP multicast addresses specify a "set" of IP hosts that have joined a group and are interested in receiving multicast traffic designated for that particular group. Hosts must be a member of the group to receive the data stream.
Hosts that are interested in receiving data flowing to a particular group must join the group using IGMP (IGMP is discussed in the "Internet Group Management Protocol (IGMP)" section later in this document). This group has no physical or geographical boundaries-the hosts can be located anywhere on the Internet or any private internetwork. A multicast group is an arbitrary group of receivers that expresses an interest in receiving a particular data stream. Multicast is based on the concept of a group. This process is further explained in the following sections. The video data stream will then be delivered only to the network segments that are in the path between the source and the receivers.
The routers use Protocol Independent Multicast (PIM) to dynamically create a multicast distribution tree. The routers are then responsible for delivering the data from the source to the receivers. The receivers indicate their interest by sending an Internet Group Management Protocol (IGMP) host report to the routers in the network. In the example shown in Figure 1, the receivers (the designated multicast group) are interested in receiving the video data stream from the source. Figure 1 shows how IP multicast is used to deliver data from one source to many interested recipients.įigure 1 Multicast Transmission to Many Receivers In these applications, IP multicast is the only way to send to more than one receiver simultaneously. High-bandwidth applications, such as MPEG video, may require a large portion of the available network bandwidth for a single stream. Even low-bandwidth applications can benefit from using Cisco IP multicast when there are thousands of receivers. Some, such as application-level multicast, require the source to send an individual copy to each receiver. Many alternatives to IP multicast require the source to send more than one copy of the data. Multicast packets are replicated in the network at the point where paths diverge by Cisco routers enabled with Protocol Independent Multicast (PIM) and other supporting multicast protocols, resulting in the most efficient delivery of data to multiple receivers. IP multicast delivers application source traffic to multiple receivers without burdening the source or the receivers while using a minimum of network bandwidth. Applications that take advantage of multicast include video conferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news. IP multicast is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to potentially thousands of corporate recipients and homes. Please refer to Beau Williamson's book titled Developing IP Multicast Networks, Volume 1 (Cisco Press, 1999) if you need more information about any of the topics presented in this overview. It is assumed that the reader is familiar with TCP/IP, Border Gateway Protocol (BGP), and networking in general.
This document is intended as a general "refresher" on IP multicast, not a tutorial. Finally, interdomain protocols are covered, such as Multiprotocol Border Gateway Protocol (MBGP), Multicast Source Directory Protocol (MSDP), and Source Specific Multicast (SSM). Then intradomain multicast protocols are reviewed, such as Internet Group Management Protocol (IGMP), Cisco Group Management Protocol (CGMP), Protocol Independent Multicast (PIM) and Pragmatic General Multicast (PGM). First, general topics such as multicast group concept, IP multicast addresses, and Layer 2 multicast addresses are discussed. This overview provides a brief, summary overview of IP Multicast. IP multicast provides a third possibility: allowing a host to send packets to a subset of all hosts as a group transmission. Traditional IP communication allows a host to send packets to a single host (unicast transmission) or to all hosts (broadcast transmission). Multicast Source Discovery Protocol (MSDP)Īll sections were updated and new sections were added.Īll sections were updated, new sections were added, and some sections were removed. Multiprotocol Border Gateway Protocol (MBGP)
Router-Port Group Management Protocol (RGMP) Multicast in the Layer 2 Switching Environment Internet Group Management Protocol (IGMP) IP Multicast Technology Overview Table Of Contents