4000 Router Series IP Multicast Configuration Guide

Figure 9 Positions of Layer 2 multicast protocols

• IGMP snooping and MLD snooping:

IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2

devices. They generate Layer 2 multicast forwarding tables by listening to IGMP or MLD messages

exchanged between the hosts and Layer 3 multicast devices, and manage and control multicast

data forwarding on demand in Layer 2 networks.

• PIM snooping and IPv6 PIM snooping:

PIM snooping and IPv6 PIM snooping run on Layer 2 devices. They work with IGMP snooping or

MLD snooping to analyze received PIM messages, and adds 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:

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, 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 to a unicast model, a multicast model is more complex in the following aspects:

• To ensure multicast packet transmission in the network, unicast routing tables, routing tables for

multicast (for example, the MBGP routing table), and static multicast routes 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