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In the traditional multicast-on-demand mode, when users in different VLANs on a Layer 2 device
need multicast information, the upstream Layer 3 device needs to forward a separate copy of the
multicast data to each VLAN of the Layer 2 device. When the multicast VLAN or IPv6 multicast VLAN
feature is enabled on the Layer 2 device, the Layer 3 multicast device sends only one copy of the
multicast data to the multicast VLAN or IPv6 multicast VLAN on the Layer 2 device. This approach
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. To deliver multicast
packets to receivers located at different positions of the network, multicast routers on the forwarding
paths 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—for example, MBGP routing table—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 undergoes 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.