550 CHAPTER 41: MULTICAST OVERVIEW
For the SSM model, multicast routes are not divided into inter-domain routes and
intra-domain routes. Since receivers know the position of the multicast source,
channels established through PIM-SM are sufficient for multicast information
transport.
Layer 2 multicast protocols
Layer 2 multicast protocols include IGMP Snooping/MLD Snooping and multicast
VLAN/IPv6 multicast VLAN. Figure 169 shows where these protocols are in the
network.
Figure 169 Position of Layer 2 multicast protocols
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.
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.
Source
Receiver Receiver
IPv4/IPv6 multicast packets
IGMP Snooping
/MLD Snooping
Multicast VLAN
/IPv6 Multicast VLAN
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