Multicast Packet Forwarding Mechanism - H3C S3610-28P Operation Manual

Operation Manual – Multicast Protocol
H3C S3610&S5510 Series Ethernet Switches
1) IGMP Snooping/MLD Snooping
Running on Layer 2 devices, Internet Group Management Protocol Snooping (IGMP
Snooping) and Multicast Listener Discovery Snooping (MLD Snooping) are multicast
constraining mechanisms that manage and control multicast groups by listening to and
analyzing IGMP or MLD messages exchanged between the hosts and Layer 3
multicast devices, thus 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. With the
multicast VLAN or IPv6 multicast VLAN feature enabled on the Layer 2 device, the
Layer 3 multicast device needs to send only one copy of multicast to the multicast
VLAN or IPv6 multicast VLAN on the Layer 2 device. This avoids waste of network
bandwidth and extra burden on the Layer 3 device.

1.4 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.
Therefore, to deliver multicast packets to receivers located in different parts of the
network, multicast routers on the forwarding path 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 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 is subject to 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.
Note:
For details about the RPF mechanism, refer to
Forwarding.
Introduction to Multicast Routing and
1-13
Chapter 1 Multicast Overview