3Com S7902E, Configuration Manual

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Multicast VLAN Configuration
When configuring multicast VLAN, go to these sections for information you are interested in:
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Introduction to Multicast VLAN
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Multicast VLAN Configuration Task List
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Configuring Sub-VLAN-Based Multicast VLAN
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Configuring Port-Based Multicast VLAN
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Displaying and Maintaining Multicast VLAN
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Multicast VLAN Configuration Examples
Introduction to Multicast VLAN
As shown in Figure 1-1 , in the traditional multicast programs-on-demand mode, when hosts, Host A,
Host B and Host C, belonging to different VLANs require multicast programs on demand service, the
Layer 3 device, Router A, needs to forward a separate copy of the multicast traffic in each user VLAN to
the Layer 2 device, Switch A. This results in not only waste of network bandwidth but also extra burden
on the Layer 3 device.
Figure 1-1 Multicast transmission without multicast VLAN
The multicast VLAN feature configured on the Layer 2 device is the solution to this issue. With the
multicast VLAN feature, the Layer 3 device needs to replicate the multicast traffic only in the multicast
VLAN instead of making a separate copy of the multicast traffic in each user VLAN. This saves the
network bandwidth and lessens the burden of the Layer 3 device.
The multicast VLAN feature can be implemented in two approaches, as described below: