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Chapter 1

IP Multicast Overview

Multicast Forwarding


Multicast Groups /22

Address Range

Scope Site-local


IP/TV High-Rate Traffic -

Restricted to local Campus

IP/TV Medium-Rate Traffic -


Restricted to 768k+ Sites

IP/TV Low-Rate Traffic -


Restricted to 256k+ Sites

Multicast Music-on-Hold -

Organization-Local No restrictions

For illustration purposes, the examples in this chapter implement IP/TV and MoH in an IP multicast environment. Table 1-1 lists the example address ranges used in these examples.

Table 1-1

Design Guide IP Multicast Address Assignment for Multicast Music-on-Hold and IP/TV

The IP/TV streams have been separated based on the bandwidth consumption of each stream. IP/TV High-Rate traffic falls into the site-local scope ( and is restricted to the local campus network. IP/TV Medium-Rate traffic falls into one range of the organization-local scope ( and is restricted to sites with bandwidth of 768 Kbps or greater. IP/TV Low-Rate traffic falls into another range of the organization-local scope ( and is restricted to sites with bandwidth of 256 Kbps or greater. Finally, multicast MoH traffic falls into yet another range of the organization-local scope ( and has no restrictions.

This type of scoping allows multicast applications to be controlled through traffic engineering methods discussed later in this chapter.


The /22 networks were subnetted from the range, allowing for four address classes. can be used for additional applications not defined in this document.

Multicast Forwarding

IP multicast delivers source traffic to multiple receivers using the least amount of network resources as possible without placing additional burden on the source or the receivers. Multicast packets are replicated in the network by Cisco routers and switches enabled with Protocol Independent Multicast (PIM) and other supporting multicast protocols.

Multicast capable routers create “distribution trees” that control the path that IP Multicast traffic takes through the network in order to deliver traffic to all receivers. PIM uses any unicast routing protocol to build data distribution trees for multicast traffic. The two basic types of multicast distribution trees are source trees and shared trees.

Source trees—The simplest form of a multicast distribution tree is a source tree with its root at the source and branches forming a tree through the network to the receivers. Because this tree uses the shortest path through the network, it is also referred to as a shortest path tree (SPT).

Shared trees—Unlike source trees that have their root at the source, shared trees use a single common root placed at some chosen point in the network. This shared root is called a Rendezvous Point (RP).

Cisco AVVID Network Infrastructure IP Multicast Design



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