ADTRAN Stub Routing User Manual

Configuration Guide

IP Multicast Stub Routing in AOS

Overview and Examples for IP Multicast Technology

IP multicast provides an efficient method for delivering common information from a single source to multiple recipients without unnecessary duplication and network resource waste. It is most often associated with the delivery of media content such as video and audio, but may also be used to deliver data such as news, stock tickers, and other one-to-many messages.

61200890L1-29. 3A

March 2005

This document provides a high-level description of IP multicast and describes the multicast features introduced in the ADTRAN Operating System (AOS) Release 7.1. For more detailed information regarding specific command syntax, refer to the AOS Command Reference Guide on

your ADTRAN OS Documentation CD.

This guide consists of the following sections:

• IP Multicast Overview on page 2

• IP Multicast Addressing and Protocols on page 6

• IP Multicast in Stub Topologies on page 8

• IP Multicast Stub Routing in the AOS on page 10

• Example Configuration on page 13

• Key Differences in Multicast Stub Routing on page 14

• Frequently Asked Questions (FAQs) on page 15

• Glossary on page16

IP Multicast Overview IP Multicast Stub Routing in AOS

IP Mult icast Overview

IP multicast has many applications, ranging from video and/or audio program delivery, music-on-hold for an IP PBX, conferencing applications, and delivery of software updates, data, or other information to multiple sites and/or devices. This document uses simple example applications to illustrate the various components of IP multicast.

Satellite Classroom Example

The following example describes a one-to-many application and compares operation in a non-multicast network to operation in a multicast network. This example is illustrated in Figure 1 on page 3 .

A university has opened satellite classrooms in several towns across a large rural portion of the country , providing local residents access to live classes. Satellite offices connect to the university backbone and include a small LAN at each location. Students participate in classes using computers connected to the satellite classroom LAN. Headsets are used since each student may be attending a different class. Classes are conducted at scheduled times from the central university campus, and the live audio and video streams are made available via the media server. To join a class, the student logs into a computer at the satellite classroom and selects a URL, opening a media player and pointing it to the appropriate content on the media server. The media server configures the media player for the content's stream format (CODECs, etc.), preparing it to receive and play the selected content.

Satellite Classroom Application on a Non-Multicast Network

Referring to Figure 1 on page 3, broadcast. Since the network is not multicast-enabled, the

PC1, PC2, PC4,andPC6 have subscribed to the same classroom

Media Server must send a separate copy of the

content to the IP address of each PC. In this case, there are four copies of the content traversing the network in four streams. Th e link from the bottleneck. In this backbone layout, the backbone path serving satellite sites 1 and 2 (

Router 1/Satellite Router 2

) is another potential bottleneck. Notice that PC1 and PC2 are on the same

Media Server to the University Backbone is a potential

Satellite

broadcast domain. Even though they are subscribed to the exact same content, that stream is transmitted twice and consumes twice the bandwidth on that segment. This solution does not scale.

IP Multicast Stub Routing in AOS IP Multicast Overview

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Figure 1. Non-Multicast Network

PCs 1, 2, 4, and 6 have subscribed to a specific media server content which is deliveredwith a separate unicast stream

for each subscribing client PC.

IP Multicast Overview IP Multicast Stub Routing in AOS

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,andPC6 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 i t is received by both PCs. This solution is much more scalable.

IP Multicast Stub Routing in AOS IP Multicast Overview

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Figure 2. Multicast-Enabled Network

PCs 1, 2, 4, and 6 have subscribed toa specific media servercontent which is delivered on asingle multicast stream

and copied once to each segment containing subscribing client PCs.

When using subinterfaces (frame relay PVCs, VLAN subinterfaces, etc.), each subinterface is a separate logical IP interface. It is possible that a multicast stream may be forwarded to each subinterface, resulting in the stream being sent multiple times over the same physical interface.

IP Multicast Addressing and Protocols IP Multicast Stub Routing in AOS

IP Mul ticast Ad dressing and Protocols

Multicast Addressing

The IPV4 address scheme (layer 3) sets aside Class D addresses for use in IPmulticast. RFC1112 discusses multicast addressing in detail. The Class D range is from 224.0.0.0 through 239.255.255.255. The range from 224.0.0.0 through 224.0.0.255 is reserved for local administrative or maintenance use and is usually limited to the scope of a single subnet. In other words, processes on devices connected to the same segment use these addresses to communicate with each other.

Examples in this range include:

• The all-hosts address (224.0.0.1)

• The all-routers address (224.0.0.2)

• Routing protocols such as RIP V2 (224.0.0.9) and OSPF (224.0.0.5 and 6)

• Multicast IProuting protocols such as DVMRP (224.0.0.4) and PIM (224.0.0.13)

The range from 224.0.1.0 through239.255.255.255 is used for IP multicast where a sourcesends contentto multiple receivers via a multicast-enabled network, as this document describes.

For layer 2, a specific range of ethernet addresses for IP multicast use has been set aside (01-00-5E-xx-xx-xx). RFC1112 discusses the technique used to map IP layer 3 multicast addresses into Ethernet layer 2 multicast addresses. Use of layer 2 multicast addresses is critical to network performance since it allows a device’s network interface to listen for a specific set of addresses in hardware instead of having to listen to all addresses and sort through them in software.

The benefit of using a multicast address instead of a broadcast address is that only devices running a process that uses a given multicast address need listen for the address. Other devices are not interrupted when a multicast address is transmitted. With broadcast addressing, all attached devices are interrupted to listen to a broadcast packet, whether they need it or not.

All devices that wish to receive the same IP multicast content are referred to as a group. The multicast IP address that a specific content is being sent to is referred to as the group address.

IGMP – Internet Group Management Protocol

The IGMP protocol allows a device to notify a directly-connected multicast router that it wishes to join a specific group and therefore receive packets sent to that group address. It also allows a router to query attached segments (subnets) to determine whether any group members remain. If no remaining group members are detected, streams to that group are no longer forwarded to that segment. In IGMP V1, when a device wishes to leave a group, it ceases to respond to the router’s query . When no devices respond on a given segment, the router stops forwarding that group to that segment. This causes some lag time between when the last device stops needing a stream and when the router stops sending the stream. To reduce this lag time and make better useof network resources, IGMP V2 introduced a specific leave message and process that expedites termination of a stream to an interface when the last member leaves.

Multicast Routing Protocols

Multicast routing is a complex topic, and its details are beyond the scope of this document. To summarize, whereas IGMP is typically used by an end device to signal a directly-connected router that it wishes to join a specific multicast group, a multicast routing protocol allows a router to pass this information on to other

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