ADTRAN Stub Routing User Manual

Page 4

IP Multicast Overview

IP Multicast Stub Routing in AOS

61200890L1-29.3A

Satellite Classroom Application on a Multicast-Enabled Network

In a multicast-enabled network, the media server sends specific content in a single stream to a specific
multicast IP address, much like a local broadcast TV station sends its content on a specific broadcast
frequency. The network has multicast intelligence and is able to make copies of the stream as necessary to
reach all active receivers.

This provides two significant efficiencies:

1. At any given point in the network there is never more than one instance of a particular content stream.

2. The network will only copy and forward a stream to locations that have active receivers.

Referring to Figure 2 on page 5,

PC1

PC2

PC4

, and

PC6

have subscribed to the same classroom

broadcast. Using IGMP, these PCs have signaled to their local router that they wish to receive this
multicast address. In multicast-speak, these PCs have joined the multicast group identified by that
multicast address. The satellite routers use a multicast routing protocol (most likely PIM) to signal other
routers in the backbone and the

Central Router

that they have receivers for that multicast group. Each

router in the network then understands if it is in the path toward receivers for that group (this description is
greatly simplified).

The

Media Server

is able to send a single copy of the stream to the multicast IP address (group address).

The

Central Router

receives this stream and sends a copy out all interfaces that have receivers for that

group. This is repeated at each router until the stream arrives at each segment that has receivers. Since
there is never more than one copy of the stream on any given link, the possibility of a bottleneck is greatly
reduced. Notice that

PC1

and

PC2

are on the same broadcast domain.

Satellite Router 1

makes a single

copy of the stream and transmits it into that broadcast domain, where it is received by both PCs. This
solution is much more scalable.