Multicast Packet Forwarding Mechanism; Multi-Instance Multicast; Introduction To The Multi-Instance Concept - 3Com MSR 50 Series Configuration Manual

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Table of Contents

Multicast Packet

Multi-Instance

Introduction to the

Multi-Instance Concept

In a multicast model, a multicast source sends information to a host group, which

is identified by a multicast group address in the destination address field of IP

multicast packets. Therefore, to deliver multicast packets to receivers located in

different parts of the network, multicast routers on the forwarding path usually

need to forward multicast packets received on one incoming interface to multiple

outgoing interfaces. Compared with a unicast model, a multicast model is more

complex in the following aspects.

To ensure multicast packet transmission in the network, unicast routing tables

or multicast routing tables specially provided for multicast must be used as

guidance for multicast forwarding.

To process the same multicast information from different peers received on

different interfaces of the same device, every multicast packet is subject to a

reverse path forwarding (RPF) check on the incoming interface. The result of

the RPF check determines whether the packet will be forwarded or discarded.

The RPF check mechanism is the basis for most multicast routing protocols to

implement multicast forwarding.

For details about the RPF mechanism, refer to

"RPF Mechanism" on page

Multi-instance multicast refers to multicast in virtual private networks (VPNs).

VPN networks need to be isolated from one another and from the public network.

through PE devices.

Figure 324 Networking diagram for VPN