IP multicast is a bandwidth-conserving technology that reduces traffic by delivering a single stream of information simultaneously to potentially thousands of businesses and homes. Applications that take advantage of multicast include video conferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news.

This module contains a technical overview of IP multicast. IP multicast is an efficient way to use network resources, especially for bandwidth-intensive services such as audio and video. Before beginning to configure IP multicast, it is important that you understand the information presented in this module.

When you troubleshoot multicast routing, the primary concern is the source address. Multicast has a concept of Reverse Path Forwarding (RPF) check. When a multicast packet arrives on an interface, the RPF process checks to ensure that this incoming interface is the outgoing interface used by unicast routing in order to reach the source of the multicast packet. This RPF check process prevents loops. Multicast routing does not forward a packet unless the source of the packet passes a RPF check. Once a packet passes this RPF check, multicast routing forwards the packet based only upon the destination address.

Like unicast routing, multicast routing has several available protocols, such as Protocol Independent Multicast dense mode (PIM-DM), PIM sparse mode (PIM-SM), Distance Vector Multicast Routing Protocol (DVMRP), Multicast Border Gateway Protocol (MBGP), and Multicast Source Discovery Protocol (MSDP). The case studies in this document walk you through the process to troubleshoot various problems. You will see which commands are used in order to quickly pinpoint the problem and learn how to resolve it. The case studies listed here are generic across the protocols, except where noted.

IP Multicast Routing Configuration Guide, Cisco IOS Release 15.2(2)E (Catalyst 3750-X and 3560-X Switches)

Traditional IP communication allows a host to send packets to a single host (unicast transmission) or to all hosts (broadcast transmission). IP multicast provides a third possibility: allowing a host to send packets to a subset of all hosts as a group transmission. This overview provides a brief, summary overview of IP Multicast. First, general topics such as multicast group concept, IP multicast addresses, and Layer 2 multicast addresses are discussed. 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). Finally, interdomain protocols are covered, such as Multiprotocol Border Gateway Protocol (MBGP), Multicast Source Directory Protocol (MSDP), and Source Specific Multicast (SSM).

This document is intended as a general "refresher" on IP multicast, not a tutorial. It is assumed that the reader is familiar with TCP/IP, Border Gateway Protocol (BGP), and networking in general. 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.

Introduction

IP multicasting is a bandwidth-conserving technology that reduces traffic because it simultaneously delivers a single stream of information to thousands of corporate recipients and homes. Applications that take advantage of multicast include video conferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news. This document discusses the basics of how to configure multicast for various networking scenarios.