R3303-HP HSR6800 Routers IP Multicast Configuration Guide

PIM snooping runs on Layer 2 devices. It determines which ports are interested in multicast data by

analyzing the received PIM messages, and add the ports to a multicast forwarding entry to make

sure multicast data can be forwarded to only the ports that are interested in the data.

• 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 feature is enabled on

the Layer 2 device, the Layer 3 multicast device sends only one copy of multicast to the multicast

VLAN on the Layer 2 device. This approach avoids wasting network bandwidth and placing an

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

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.

For more information about the RPF mechanism, see "Configuring multicast routing and forwarding" and

"Configuring IPv6 multicast routing and forwarding."

Multicast support for VPNs

Multicast support for VPNs refers to multicast applied on virtual private networks (VPNs).

Multicast support for VPNs is not available on IPv6 networks.

Introduction to VPN instances

VPNs must be isolated from one another and from the public network. As shown in Figure 10, VPN A and

VPN B separately access the public network through PE devices.