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76.1 Multicast Overview
Traditionally, IP packets are transmitted in one of either two ways - Unicast (One sender to one recipient)
or Broadcast (One sender to everybody on the network). Multicast delivers IP packets to just a group of
hosts on the network.
Internet Group Management Protocol (IGMP) is a network-layer protocol used to establish membership
in a multicast group - it is not used to carry user data. Refer to RFC 1112, RFC 2236 and RFC 3376 for
information on IGMP versions 1, 2 and 3 respectively.
76.1.1 IP Multicast Addresses
In IPv4, a multicast address allows a device to send packets to a specific group of hosts (Multicast
group) in a different subnetwork. A multicast IP address represents a traffic receiving group, not
individual receiving devices. IP addresses in the Class D range (224.0.0.0 to 239.255.255.255) are used for
IP multicasting. Certain IP multicast numbers are reserved by IANA for special purposes (See the IANA
website for more information).
76.1.2 IGMP Filtering
With the IGMP filtering feature, you can control which IGMP groups a subscriber on a port can join. This
allows you to control the distribution of multicast services (Such as content information distribution)
based on service plans and types of subscription. You can set the OLT to filter the multicast group join
reports on a per-port basis by configuring a multicast channel and associating it to the IGMP channel of
a uniport of the ONT.
76.1.3 IGMP Snooping
The OLT can passively snoop on IGMP packets transferred between IP multicast routers/switches and IP
multicast hosts to learn the IP multicast group membership. It checks IGMP packets passing through it,
picks out the group registration information, and configures multicasting accordingly. IGMP snooping
allows the OLT to learn multicast groups without you having to manually configure them.
The OLT forwards multicast traffic destined for multicast groups (That it has learned from IGMP snooping
or that you have manually configured) to ports that are members of that group. IGMP snooping
generates no additional network traffic, allowing you to significantly reduce multicast traffic passing
through your OLT.
Chapter 76 Multicast
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