Chapter 7: IP Multicast
In Depth
Over time, our use of computers has moved from the local desktop to the local network. Most of this network
traffic consists of using local network sources, such as printing and file transfers. As our personal computers
become more powerful, the applications that we use now contain more audio and video components. We’ve
now started to move this traffic onto the network.
Today, it is still the norm for network traffic to be between one sender and one recipient. That is slowly
changing. As different network resources become available, new resources such as messaging, multimedia,
distance learning, and Internet access are causing a large increase in data traffic. This type of traffic usually
involves one server sending a data stream to multiple users; a good example of this would be video
conferencing and software updates in the intranet.
This type of traffic can be very demanding in terms of data usage. For example, if 100 people want a 1.5MB
file, the result is a demand for more than 150MB of data−link usage. Even on a T3, that’s a lot of
simultaneous use. What’s more, this calculation doesn’t take into account the rest of the users’ applications
and data requirements.
One way to provide users with this high−bandwidth information and at the same time minimize the traffic on
the network is to utilize IP multicast. IP multicast enables data to be sent once and received by all the
recipients that requested it.
The concept behind IP multicasting is that end recipients join a multicast group. The information that is
requested is then delivered to all members of that group by the network infrastructure. The sender of the data
doesn’t need to know anything about the recipients. In this manner, only one copy of a multicast message will
pass over any link in the network, and copies of the message will be made only where the paths diverge. This
is a much more effective method of delivering traffic destined for multiple locations, and it provides
significant performance improvements for the network.
In this chapter, we will explain the concepts behind IP multicasting. We will cover the types of multicast
traffic and introduce you to the way multimedia traffic types are routed on the network. Finally, we will look
at the methods to configure IP routing on your Catalyst switches and how to manage the resulting multicast
traffic. We will begin with a discussion of the different types of multicast traffic.

IP Multicasting Overview

IP multicasting is an extension of the standard IP protocol and is described in RFC 1112, “Host Extensions for
IP Multicasting.” IP multicasting is the transmission of an IP datagram to a group identified by a single IP
destination address. A multicast datagram is delivered to all members of its destination host group using User
Datagram Protocol (UDP). Membership in these groups is unrestricted—hosts can be members of multiple
groups, and they may join or leave at any time.
IP multicast datagrams are handled by multicast routers. A host transmits an IP multicast datagram as a local
network multicast that reaches a multicast router. The router examines the packet and begins to provide the
host with the requested multicast traffic. If the router is not receiving the requested multicast traffic, it will
pass the request to other multicast routers.
IP traffic can travel the network in one of three ways:
Broadcast•
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