Multicast; Overview; Igmp Configuration - Dell PowerConnect M6220 Configuration Manual

Multicast

Overview

IP Multicasting enables a network host (or multiple hosts) to send an IP datagram to multiple
destinations simultaneously. The initiating host sends each multicast datagram only once to a
destination multicast group address, and multicast routers forward the datagram only to hosts who
are members of the multicast group. Multicast enables efficient use of network bandwidth, as each
multicast datagram needs to be transmitted only once on each network link, regardless of the
number of destination hosts. Multicasting contrasts with IP unicasting, which sends a separate
datagram to each recipient host.
Hosts must have a way to identify their interest in joining any particular multicast group, and routers
must have a way to collect and maintain group memberships: these functions are handled by the
IGMP protocol in IPv4. In IPv6, multicast routers use the Multicast Listener Discover (MLD)
protocol to maintain group membership information.
Multicast routers must also be able to construct a multicast distribution tree that enables forwarding
multicast datagrams only on the links that are required to reach a destination group member.
Protocols such as DVMRP, and PIM handle this function.
This section describes the following multicast protocols:
• "IGMP Configuration" on page 119
• "IGMP Proxy" on page 120
• "DVMRP" on page 122
• "PIM" on page 123

IGMP Configuration

The Internet Group Management Protocol (IGMP) is used by IPv4 hosts to send requests to join (or
leave) multicast groups so that they receive (or discontinue receiving) packets sent to those groups.
In IPv4 multicast networks, multicast routers are configured with IGMP so that they can receive join
and leave request from directly-connected hosts. They use this information to build a multicast
forwarding table.
IPv6 multicast routers use the MLD protocol to perform the functions that IGMP performs in IPv4
networks.
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Multicast