3Com S7902E, Configuration Manual

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data to each VLAN of the Layer 2 device. With the multicast VLAN or IPv6 multicast VLAN feature
enabled on the Layer 2 device, the Layer 3 multicast device needs to send only one copy of multicast to
the multicast VLAN or IPv6 multicast VLAN on the Layer 2 device. This avoids waste of network
bandwidth and extra burden on the Layer 3 device.
Multicast Packet Forwarding Mechanism
In a multicast model, a multicast source sends information to the host group identified by the 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.
z
To ensure multicast packet transmission in the network, unicast routing tables or multicast routing
tables (for example, the MBGP routing table) specially provided for multicast must be used as
guidance for multicast forwarding.
z
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 Multicast Routing and Forwarding Configuration or IPv6
Multicast Routing and Forwarding Configuration in the IP Multicast Volume .
Multi-Instance Multicast
Multi-instance multicast refers to multicast in virtual private networks (VPNs).
Introduction to the Multi-Instance Concept
VPN networks need to be isolated from one another and from the public network. As shown in Figure
1-10 , VPN A and VPN B separately access the public network through PE devices.