H3C Technologies H3C S10500 Series Switches - Manual (Page 26)

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Figure 9 Positions of Layer 2 multicast protocols
1. IGMP snooping and MLD snooping
IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2 devices.
They manage and control multicast groups by monitoring 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. PIM snooping and IPv6 PIM snooping
PIM snooping and IPv6 PIM snooping run on Layer 2 devices. They determine which ports are interested
in multicast data by analyzing the received IPv6 PIM messages, and add the ports to a multicast
forwarding entry to ensure that multicast data can be forwarded to only the ports that are interested in
the data.
3. Multicast VLAN and 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 must 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 multicast 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 that an incoming interface receives 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, the 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