This document discusses multicasting and multicast routing protocols. It begins with an introduction to multicasting versus unicasting. It then discusses multicast trees, including source-based trees and group-shared trees. It provides overviews of several multicast routing protocols: DVMRP, a distance-vector protocol; MOSPF, an extension of OSPF; and CBT, which uses a rendezvous-point tree with a core router. Key concepts like reverse path forwarding and pruning/grafting to support dynamic membership are also summarized.

1. ©The McGraw-Hill Companies, Inc., 2000 © Adapted for use at JMU by Mohamed Aboutabl, 2003
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Chapter 14
Multicasting And Multicast
Routing Protocols
 INTRODUCTION
 MULTICAST ROUTING
 MULTICAST TREES
 MULTICAST ROUTING PROTOCOLS
 DVMRP
 MOSPF
 CBT
 PIM
 MBONE

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Figure 14-1
In unicast routing, the router forwards
the received packet through
only one of its interfaces.
14.1 Introduction: Unicasting

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Figure 14-2
Multicasting
In multicast routing,
the router may forward the
received packet
through several of its interfaces.

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Figure 14-3
Multicasting versus multiple unicasting
Emulation of multicasting through
multiple unicasting is not
efficient and may
create long delays,
particularly with a large group.

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Application of Multicasting
 Access to Distributed Databases
 Information Dissemination: e.g. multicast software updates to
customers
 News Delivery
 Teleconferencing, Web Seminars
 Distant Learning

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14.3 Multicast Routing
Objectives
 Every member receives EXACTLY ONE copy of the packet
 Non-members receive nothing
 No loops in route
 Optimal path from source to each destination.
Terminology
 Spanning Tree: Source is the root, group members are the leaves.
 Shortest Path Spanning Tree: Each path from root to a leaf is the
shortest according to some metric

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14.3 Multicast Trees
 Source-Based Tree:
 For each combination of (source , group), there is a shortest path
spanning tree.
 Approach 1: DVMRP; an extension of unicast distance vector
routing (e.g. RIP)
 Approach 2: MOSPF; an extension of unicast link state routing
(e.g. OSPF)
 Group-Share Tree
 One tree for the entire group
 Rendezvous-Point Tree: one router is the center of the group and
therefore the root of the tree.
 CBT and PIM-SP protocols.

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Figure 14-4
14.4 Multicast routing protocols

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14.5 Distance Vector Multicast Routing Protocol - DVMRP
 No pre-defined route from source to destination. Tree is gradually
created by successive routers along the path.
 Uses shortest path (fewest hops)
 Prevent loops: apply Reverse Path Forwarding (RFP)
 Prevent Duplication: apply Reverse Path Broadcasting (RPB)
 Multicast with dynamic membership: apply Reverse Path
Multicasting (RPM) with pruning, grafting, and lifetime.

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Figure 14-5
Reverse Path Forwarding
In reverse path forwarding (RPF),
the router forwards only
the packets that have traveled the
shortest path from the source
to the router; all other
copies are discarded. No Loops

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Figure 14-6
Reverse Path Broadcasting
Prevent Duplication in RPF

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Figure 14-7
RPF versus RPB
 The router with the shortest path to the source becomes the
designated parent of a network
 A Router forwards packets only to its designated child networks

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RPB creates a shortest path
broadcast tree from the source
to each destination.
It guarantees that each destination
receives one and only
one copy of the packet.

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Figure 14-8
RPF, RPB, and RPM
RPM adds pruning and grafting to RPB
to create a multicast shortest
path tree that supports
dynamic membership changes.

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MOSPF
14.6

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Figure 14-9
Unicast tree and multicast tree

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Core-Based Tree
CBT
14.7

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Figure 14-10
Shared-group tree with rendezvous router

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Figure 14-11
Sending a multicast packet to
the rendezvous router

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In CBT, the source sends the
multicast packet (encapsulated in a
unicast packet) to the core router.
The core router decapsulates the
packet and forwards it
to all interested hosts.