Even multicasts from the customer are wrapped in a unicast
outer destination MAC address because there is only one I-component that needs to
receive it.
Backbone bridges have the same ability to prune B-VLANs and multicast MAC addresses
that were available to provider bridges. Note, however, that IGMP snooping
plays no part in the operation of the B-components. This is because each EVC??™s multicast
needs are independent of every other EVC??™s needs. Instead, the backbone bridges
have available the Multiple Multicast Registration Protocol (MMRP). This protocol is
similar to MVRP, but it registers multicast MAC addresses instead of VLANs. This allows
the backbone network administrator to assign a multicast MAC address/B-VLAN
to each EVC and allows each I-component to register with the B-components those
{multicast address, B-VLAN} pairs that it needs to receive. Then, no multicast is delivered
across the backbone to any I-component that does not need to receive it. Of course,
the administrator is free to reduce the amount of work done by MVRP and MMRP and
accept excess deliveries of multicasts in the backbone network. The administrator can
trade control plane activity for data plane activity.
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