Multicast Packet Forwarding Mechanism - HP 6125XLG Ip Multicast Configuration Manual

Figure 9 Positions of Layer 2 multicast protocols
IGMP snooping and MLD snooping:
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IGMP snooping and MLD snooping run on Layer 2 devices as multicast constraining mechanisms
to improve multicast forwarding efficiency. They generate Layer 2 multicast forwarding tables by
listening to IGMP or MLD messages exchanged between the hosts and Layer 3 multicast devices.
This effectively controls the flooding of multicast data in Layer 2 networks.
PIM snooping and IPv6 PIM snooping:
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PIM snooping and IPv6 PIM snooping run on Layer 2 devices. They work with IGMP snooping or
MLD snooping to analyze received PIM messages. Then, they add the ports that are interested in
specific multicast data to a PIM snooping routing entry or IPv6 PIM snooping routing entry. In this
way, multicast data can be forwarded to only the ports that are interested in the data.
Multicast VLAN and IPv6 multicast VLAN:
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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 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 method
avoids waste of network bandwidth and extra burden on the Layer 3 device.

Multicast packet forwarding mechanism

In a multicast model, multicast receivers of a multicast group are usually located at different positions of
the network. They are identified by the same multicast group address. To deliver multicast packets to
these receivers, a multicast source encapsulates the multicast data in an IP packet with the multicast group
address as the destination address. Multicast routers on the forwarding paths forward multicast packets
that an incoming interface receives through multiple outgoing interfaces. Compared to a unicast model,
a multicast model is more complex in the following aspects:
To ensure multicast packet transmission in the network, different routing tables are used for multicast
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forwarding. These routing tables include unicast routing tables, routing tables for multicast (for
example, the MBGP routing table), and static multicast routing tables.
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