H3C Technologies H3C S3100V2 Series Switches - Manual (Page 19)

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Figure 9 Positions of Layer 2 multicast protocols
Source
Receiver Receiver
IPv4/IPv6 multicast packets
IGMP Snooping
/MLD Snooping
Multicast VLAN
/IPv6 Multicast VLAN
1. IGMP snooping/MLD snooping
IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2 devices.
They manage and control multicast groups by listening to and analyzing IGMP or MLD messages
exchanged between the hosts and Layer 3 multicast devices, effectively controlling the flooding of
multicast data in a Layer 2 network.
2. Multicast VLAN/IPv6 multicast VLAN
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.