Chapter 10: Igmp Configuration; Igmp Overview; About Ip Multicast Group Management - Enterasys G3G170-24 Cli Reference Manual

This chapter describes the IGMP Configuration set of commands and how to use them.
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IGMP Overview

Configuring IGMP at Layer 2
Configuring IGMP on Routing Interfaces
IGMP Overview

About IP Multicast Group Management

The Internet Group Management Protocol (IGMP) runs between hosts and their immediately
neighboring multicast device. The protocol's mechanisms allow a host to inform its local device
that it wants to receive transmissions addressed to a specific multicast group.
A multicast‐enabled device can periodically ask its hosts if they want to receive multicast traffic. If
there is more than one device on the LAN performing IP multicasting, one of these devices is
elected "querier" and assumes the responsibility of querying the LAN for group members.
Based on the group membership information learned from IGMP, a device can determine which
(if any) multicast traffic needs to be forwarded to each of its ports. At Layer‐3, multicast devices
use this information, along with a multicast routing protocol, to support IP multicasting across an
IP network.
IGMP provides the final step in an IP multicast packet delivery service, since it is only concerned
with forwarding multicast traffic from the local device to group members on a directly attached
subnetwork or LAN segment.
This device supports IP multicast group management by passively snooping on the IGMP query
and IGMP report packets transferred between IP multicast devices and IP multicast host groups to
learn IP multicast group members.
The purpose of IP multicast group management is to optimize a switched network's performance
so multicast packets will only be forwarded to those ports containing multicast group hosts or
multicast devices instead of flooding to all ports in the subnet (VLAN).
In addition to passively monitoring IGMP query and report messages, the G‐Series can also
actively send IGMP query messages to learn locations of multicast devices and member hosts in
multicast groups within each VLAN.
However, note that IGMP neither alters nor routes any IP multicast packets. Since IGMP is not
concerned with the delivery of IP multicast packets across subnetworks, an external IP multicast
device is needed if IP multicast packets have to be routed across different subnetworks.
IGMP Configuration
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