35 IP Multicast Routing
Configuration
35.1 Overview
This chapter describes how to configure multicast routing protocols. For a complete
description of the IP multicast routing commands, please refer to other chapters IP Multicast
Routing Commands.
Traditional IP transmission only allows packet transmission from one host to a single host
(unicast communication) or to all hosts (broadcast communication). IP multicast provides a
third scheme, allowing packet transmission from one host to a subset of all hosts. These
hosts are known as group members.
The destination address sent to the group member is a Class D IP address which can be in
the range from 224.0.0.0 to 239.255.255.255. Transmission of multicast packets is similar to
UDP, which is a best-effort service. It does not provide reliable transmission and error control
like TCP.
The multicast application consists of senders and receivers. The sender, regardless of
whether it is a member of a group, can send multicast packets. However, it‘s required to join
to certain group in advance for the receiver to receive the packets of this group.
Membership in a multicast group is dynamic; hosts can join and leave at any time. There is
no restriction on the location or number of members in a multicast group. If necessary, a host
can be a member of more than one multicast group at a time. Therefore, the active state of
groups and number of group members can vary with the time.
Routers maintain routing tables to forward multicast packets through a multicast routing
protocol (such as PIM-DM and PIM-SM) and learn the status of the members within a group
on their directly attached subnets through the IGMP. A host can join in a certain IGMP group
by sending IGMP Report packet.
IP multicast is ideal for ―one-to-multiple‖ multimedia applications.
35.1.1 IP Multicast Routing Implementation
Multicast routing is composed of the following protocols in router software:
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