User's Manual

Table Of Contents

Base Unit User Guide Issue 1 Tech-X Flex™

5-26 5: IP and Video Testing

Intro

Wi-Fi

10/100

System

IP/Video

Specs

Figure 5-8 Hypothetical multicast network with multicasting and IGMP

In this example, the routers are multicast-aware and can make intelligent decisions about packet

forwarding. The routers control the forwarding of multicast packets, with those routers directly connected

to multicast group members using Internet Group Management Protocol (IGMP) to manage the

duplication and forwarding of packets to individual group members.

In a multicast-enabled network, multicast routers interact and dynamically maintain a logical tree for

routing multicast packets, in order to efficiently deliver the required packets to each subnet that requests

them. If no subscribers on a particular subnet are members of a given multicast group (for example, no

one on a particular subnet is viewing a particular audio/video stream), the network may automatically

adjust to avoid multicasting that stream to that subnet. Similarly, when a host on a subnet successfully

joins a group, the network will dynamically extend a branch of the respective multicast tree to the router

serving the host. In summary, therefore, multicasting improves transport efficiency both by eliminating

redundant packets from the same media source, and by eliminating the indiscriminate broadcast of any

packets to branches in the network that have no hosts requesting them.

Note that multicasting is a form of “selective broadcasting,” where packets from the source are simply

duplicated as necessary and forwarded onto the respective links, all the way down the multicast tree to

each requesting group member. IP multicast routers use specialized multicast routing protocols such as